Can Sim A cap both branches of one career and have that count as 2 individual capped careers?
Not a member of the challenge team here, but I'll try and answer this anyway. Well, I'm pretty sure that a single Sim can't switch between branches of the same career (I may be wrong on this though, I haven't checked, just remembered reading it somewhere). Once you choose a branch, there's no going back. You'd have to have a seperate Sim complete the other branch (which sucks).
However, if switching branches was possible I don't think there'd be a problem because it doesn't say anything in the rules about it.