apparently
Disneyland has been running out of some droid parts. I don't know about exactly when Disneyland started running out, what WDW's stocks are or how far from opening day you want to build droids. So there's no way to tell if having a later reservation would corrrelate to the parts you want not being available. But it's all I can think of.
Party of two I don't think it matters; you can have one member of the party build and the other watch, and then the other party member builds while the one who has already built watches, or you can try to have both reservations around the same time.