Watching the American Shakespeare Co's production of The Alchemist, I was reminded of just how sublime a doubled show that features characters in disguise can be. Until the disguise is revealed, you don't know if you're looking at a whole new character or a character in disguise. The playwright provides the actors a way of putting one over on the audience at the same time they put one over on the the other characters. I think the same would be true of Shakespeare's works if we didn't already know them so well, but Jonson is just obscure enough to be able to pull off this illusion.
The Merry Devil is, of course, replete with both double-cast actors and characters in disguise, so I think we'll be able to pull the same thing off. Keeping the audience guessing as to who is who and what is real is a great way of keeping them interested. I was about to say something grossly incorrect about how we don't have this convention anymore in modern theatre, but that isn't necessarily so. Gilles Segal's The Puppetmaster of Lodz features a "third man" character; a single actor plays many roles throughout the course of the production, but late in the play it is revealed that it is a single character playing many roles. Familiarity breeds comfort, and the revelation of a character in disguise, whom we had mistaken for an actor playing another role, is one of those things that can keep an audience guessing right up through the end.