Ayckbourn at the double

I nearly walked past Sir Alan Ayckbourn, in earnest conversation with Michael Frayn ("a covey of playwrights," said Ayckbourn) backstage at the National Theatre. Until recently he tended to be - how shall I say? - on the generous side of portly, but after the loss of an impressive number of stone this is a new slimline Ayckbourn, natty in T-shirt and designer trousers. And whereas most people become distinctly ratty after stringent dieting, he seems bright-eyed and beamingly enthusiastic about work in progress.

He may be trying to set his current cast an example; they'll certainly need to be superfit for this typically individual new venture. House and Garden comprises two plays, performed simultaneously in two of the National's auditoriums. By the same cast. This includes a number of those involved in the play's premiere at Ayckbourn's Scarborough theatre base last year, some of his London regulars (such as Jane Asher), plus some intriguing Ayckbourn virgins - David Haig and Michael Siberry, who should be ideally cast as contrastingly classic examples of homo Ayckbournis, haplessly at sea with women.

House (Lyttelton), which Ayckbourn describes as "a little metronome", is - superficially, at least - the most conventional of the two, with French windows, plump chintz sofas and sherry, albeit at an unusually early hour. Businessman Teddy Platt, in the family pile, is planning a political future and hoping to impress his old schoolchum Gavin Ryng-Mayne, a smoothie popular novelist-cum-political fixer. It is the day of the village fète and Teddy has to rise above a number of problems, while his wife Trish is devising a last resort for coping with his infidelities.

Garden, structured round preparations for the fète, is ideally suited to the perennially demanding Olivier stage. Gardens in Ayckbourn rarely have benevolent fairies at their bottoms. From his very first hit, Relatively Speaking in 1965, where a Home Counties patio reveals devious deceptions under the surface drollery, through Round and Round the Garden (last of The Norman Conquests trilogy), and many others of his 58 plays, gardens are often unnerving and dangerous places where truths cloaked indoors become unfortunately aired.

There will still be the usual moans that two of the National's stages will be occupied by "commercial" plays. Comedies at that. In fact, on paper no idea could have originally seemed less commercial. Indeed, Ayckbourn told the Scarborough cast from the outset that it was "a one-off". As he said, "what two West End adjacent theatres could be miraculously simultaneously available?" Only when Trevor Nunn caught the plays in Scarborough and realised how well the experience could work at the National did the current restaging look possible.

Ayckbourn at the National reminds me of Orson Welles on Hollywood - "the biggest train-set in the world". He's having a ball with the increased resources: understudies, a village band, and more children mean that the fète side of the play (spilling over into the foyers after the show) is altogether more vital.

But although on a larger scale, the basic premise remains the same. Ayckbourn, after all, describes himself as "a director who writes occasionally" rather than as simply a dramatist. For the best part of each year he continues as artistic director of the Stephen Joseph Theatre in Scarborough where his leadership is a shining exception to the bureaucracy-led theatre culture that has stealthily taken over regional theatres.

His mentor, Stephen Joseph, had the radical conviction that "every theatre organisation should be blown up every seven years". Ayckbourn has a similar fear of staleness: "If you run a theatre all the time it can get routine." Which is why, every so often, he comes up with "event theatre" - the two-hander Intimate Exchanges sequence or the double plays of Revengers' Comedies and Sisterly Feelings. They all use their spaces in different ways, testing the limits of what theatre can do.

House and Garden need a building with a common foyer area to work properly. The challenges for the actors are legion - running from one theatre to another gives little time to adjust to different audience temperatures, acoustics or the disciplines of very contrasted playing spaces. Some characters loom large in one play, and briefly in the other ("well, we're all walk-ons in other people's lives," says Ayckbourn), while others have lightning sprints between theatres.

With teams of runners, separate stage managers on headphones to each other and CCTV in the wings, it's unlikely anyone will relive Beryl Reid's nightmare National experience. Dressed in full 18th century Venetian finery for a Goldoni play in the Olivier, she lost her way in the backstage corridors only to emerge in the bright lights of Elsinore in Hamlet on the Lyttelton stage.

Ayckbourn goes into technical rehearsals like a military campaigner; I expect it will all fall into place with ease - and that by the end of the run the cast will be fit enough for the London Marathon. The only things I haven't quite worked out are what happens if an actor dries or skips a page or two (best not to ask) and how, assuming both plays finish at approximately the same time, he can orchestrate two separate curtain calls. In two theatres. With the same actors. But there will be a solution to that too, I'm sure.