Danger - heavy traffic ahead

"Giles goes out through the French windows." When you read the script, this stage direction from Act Two, Scene One of Alan Ayckbourn's House seems perfectly innocuous. Under normal circumstances, the actor cast as Giles Mace, a mild-mannered country GP, would exit as required, head for the dressing room and wait around for his next entrance, some 15 minutes later. But circumstances at the National Theatre in London, where House is about to open are far from normal.

"Giles goes out through the French windows." When you read the script, this stage direction from Act Two, Scene One of Alan Ayckbourn's House seems perfectly innocuous. Under normal circumstances, the actor cast as Giles Mace, a mild-mannered country GP, would exit as required, head for the dressing room and wait around for his next entrance, some 15 minutes later. But circumstances at the National Theatre in London, where House is about to open are far from normal.

"Giles goes out through the French windows" is now the cue for actor Michael Siberry to leave the Lyttelton auditorium and begin a race against time prompting a cryptic walkie-talkie exchange between assistant stage manager (ASM) Lesley Walmsley and co-ordinating stage manager Courtney Bryant.

Walmsley: "Where's Michael?"

Bryant: "On Olivier staircase."

Walmsley: "Good. We won't be needing emergency dog."

Seconds later, Siberry strides onto the Olivier stage and into Act One, Scene Two of another Ayckbourn comedy, Garden, just in time for a heart-to-heart between Giles and his teenage son, Jake.

Confused? You might well be, given that soon after the start of last Friday's dress rehearsal, even Bryant, who's stage-managed at the National for more than 20 years, said, only half in jest: "I've no idea which theatre I'm in." Once he has explained how Ayckbourn's masterful stagecraft links the two plays, however, even references to "emergency dog" make perfect sense.

House and Garden are self-contained comedies, but both involve the same 14 actors, both are set on an August Saturday at the home of adulterous businessman Teddy Platt, and their action is both simultaneous and simultaneously performed; seeing one poses many questions about the characters that can only be satisfactorily answered by seeing the other. In the Lyttelton, Roger Glossop's House set represents the summer drawing room and garden terrace of Teddy's grand abode; in the Olivier, his Garden design incorporates the stone steps connecting that terrace to a large lawn. The distance between these two points would in real life be perhaps 20 yards; in the backstage warren of the National it's about five times as long.

When Giles exits through the French windows in House, Siberry, like the rest of the cast, must follow the yellow tape road: a line of thin plastic stuck onto lino floor tiles and stairs to mark the actors' route between the two auditoria. Accompanied by one of the Lyttelton ASMs, the performers go through a door, up four flights of nine stairs, through another door and along a corridor. Then it's up two more flights and through another door, at which point the Lyttelton ASM hands over to Bryant and heads back to House. At the end of another corridor, the fifth and final door leads into the Olivier for a climb up two flights of portable stairs to the rear of the set scaffold. Somewhat disoriented, the actors find themselves facing the Olivier stalls.

The 130 minutes of simultaneous action involve no less than 36 of these Lyttelton-Olivier or Olivier-Lyttelton crossovers, and seven entrance/exit points. As Ayckbourn's plotting thickens, with Teddy's hopes of a political career in House, and preparations for a summer fête in Garden disrupted by a French film actress and the collapse of three marriages, the backstage traffic becomes increasingly frantic. At "rush hour", 14 principals share the staircases with six non-speaking characters, 15 children and their chaperones, a mere nothing compared to what happens at the end. The dual curtain calls are, says Bryant, "complete hysteria".

"I've instigated a strict 'keep left' policy," he explains, "because I don't want to end up with a French farce backstage: people colliding and pushing against the same swing door from opposite sides. Every second counts. The actors have to walk as fast as possible without actually running, and the tightest crossovers need to take no more than one minute and 20 seconds." With Bryant leading the way, and both of us taking the stairs two at a time, he and I clocked 65 seconds for the House to Garden run, leaving a safety cushion of around 15 seconds (longer for the "downhill" Olivier-Lyttelton journey).

"It's tight, but from a stage management point of view, rehearsals have not been as complex as we first thought, largely because Alan's written the plays so brilliantly to keep them in sync," adds Bryant, who has worked on seven Ayckbourn-directed productions. "Alan understands the technical side of things far better than some directors, and had effectively done a lot of our work for us before rehearsals started."

Staging House/ Garden at the National is nonetheless a far trickier proposition than during the premiÿre at Ayckbourn's base, the Stephen Joseph Theatre, Scarborough, in June last year. Alison Fowler, that venue's production manager, says: "The journey between The Round theatre entrance for Garden, and the McCarthy theatre entrance for House was about 10 metres, including two doors and eight steps. I'm sure that knowing the distance was so short helped convince Alan that simultaneous staging could work."

The greater distances at the National have led to classic examples of a dramatist needing to rewrite for logistical rather than artistic reasons. In Act One of House, Teddy's wife, Trish (Jane Asher), now takes a completely inconsequential phone call, inserted, says Bryant, simply to give Siberry more time to reach the Olivier. For a similar reason, Ayckbourn has brought forward a monologue delivered by Teddy (David Haig) in House by approximately 20 minutes. It's still a very funny speech, although considering what the audience knows about Teddy at this point, it's arguably less effective than in its original position in the pre-production script.

Rewrites are one way to build in sufficient time for crossovers. But what will happen if a longer-than-average costume change or a mislaid prop delays an actor's scheduled entrance? Various "fill-ins" have been pre-arranged, says Bryant, including supplementary stage business, and short bursts from the aforementioned "emergency dog": a sound effect of Spoof, Teddy's pet hound, barking offstage.

Such precautions are all well and good in a controlled rehearsal environment; the real test will only arrive when the cast face a paying audience. "The art of comedy is being able to time your performance to the laughter," Bryant says, "taking things slower or faster according to audience response. But one Lyttelton audience could be rocking with laughter at House while the Olivier audience is hating Garden, or vice versa, and if that happens there's no leeway for the actors to pause or slow down, or they'll fall behind the other play."

In the interests of synchronicity, life for everyone would be simpler if nobody laughed. At an Ayckbourn comedy, that prospect is a non-starter. However, audience response to House/Garden's unconventional epilogue is harder to predict.

After the curtain calls, the action will continue for 45 minutes in the Lyttelton foyer, currently home to an impressive mock-up of a village fête. "There's some scripted dialogue and a raffle, and the actors are supposed to remain in character throughout," says Bryant. "It's going to be quite tricky for them and for the stage management, who'll be standing by. On stage and backstage it's our world, and there's that brilliant barrier between us and the audience; with the fête there's no barrier. Having said that, I love working on something like this, or a large musical with lots of scene changes. Looking after a two-hander would probably drive me up the wall. You need something that stretches you, otherwise you go through the motions."

He exudes seen-it-all calm, while remaining acutely aware that unexpected pitfalls may be inevitable. "The worst-case scenario is having to stop one show mid-performance for technical reasons, such as the lighting computer crashing, because we would obviously have to stop the other play as well. At least we have fixed sets, with none of the stage hydraulics which can halt big West End musicals as often as once a week."

His greatest fear is that in a bid to earn themselves a place in theatrical anecdote heaven, pranksters will attempt sabotage by re-routing the yellow tape and rearranging the signs reading "To the garden" or "To the house". If that happened, could Ayckbourn's cast suddenly find themselves in the Cottesloe, making an impromptu appearance in Arthur Miller's All My Sons? "Impossible, the Cottesloe's too far away and too separate," Bryant insists. "Although I say that with my fingers crossed."

'House' and 'Garden' are in preview at the National Theatre (020-7452 3000) and open on 9 August