I wonder how they do box-office wise. Obviously there have been some recent successes like HDM and Coast of Utopia (though I definitely remember complaints because there were no matinees of HDM Part 2 even though it was a children's show) but a recent Ayckbourn one didn't do so well in the WE and perfomances of part 2 were cancelled. It must be a disaster if they don't sell and you have, in effect, two flops instead of one.
I'm guessing the Ayckbourn you're referring to here (given the circumstances mentioned) was actually the trilogy known as Damsels in Distress. All three were connected by being set in the same location with the same group of actors playing different roles in each. All three were meant to play in rep but the London producers promoted one part (RolePlay) at the expense of the other two and then seemed surprised when the the remaining two thirds lost audiences and they ended up cancelling them -Ayckbourn was furious and since then (2002) he has not sanctioned any West End productions of his more recent work.
Another of his which does fit the two part category is The Revengers Comedies