I can say that chemistry between the girls is SUPER important. Honestly, if you want to get your money's worth, you need two girls who work together. Even if you're not into seeing the girls enjoy each other, there WILL be some interaction between them, and a two-mouth BJ is great...or so I've heard. Now, that doesn't mean I'd have to pick the girl myself, but I'd definitely need to approve any suggestions, and either meet the girl or chat with her ahead of time to make sure we know each other's boundaries.
I'd suggest starting with a drink or two to loosen everyone up, especially if its your first time with two girls. Plus it builds the sexual tension....
I think it's important to be relaxed about it, and not set too many expectations. Of course, if you really want to try a certain position, tell the ladies ahead of time.
I've never tried the female condoms in a threesome before, but that's probably the easiest way to handle that issue. Or just focus on one lady at a time to minimize the downtime.
Also, don't feel a huge pressure to perform. One play partner I had tons of group sex with used to get performance anxiety and had a hard time maintaining an erection. It was really no big deal - and if you're dealing with girls who have chemistry, it won't be a downer.