I'm sorry you're in this situation. Flaky friends are always difficult to deal with. Unfortunately, if you aren't willing to confront him to get a definite yes or no, I really don't think you can double book Saturday. True, it could work out if he flakes on you at the last minute, but you also run the risk of having conflicting plans. There's really no way to get out of that situation without potentially hurting someone. You could always say something like, "Gee, Flake, since I still haven't heard from you about Saturday, maybe we can just reschedule for a night/lunch/whatever when you know you'll be free." He'll probably take the out.