I saw Black Comedy done by a Philadelphia theater company 15 or more years ago, and instead of prefacing it with White Lies (or The White Liars, its revision), they used Tom Stoppard's The Real Inspector Hound. As they pointed out, the latter uses not only the exact same size of cast, but the same "types" within it, so the same group of actors can easily play both. It made for a very enjoyable evening for me, as I love both plays. The downside is for the actors and director -- the Stoppard is by no means a quick curtain-raiser, it takes as much time and care to prepare as the Shaffer, so it doubles their workload in rehearsal. I suppose that's one reason why the pairing hasn't been tried more often.