sure making two api calls in one ajax method is possible. Until one cant get a response from a borked node and they both fail or crafty try-catch coding alleviates that pain point.

But that means a duplicate post has to be on both chains, and somehow identifiable as such, or else what am I submitting two votes to?

Take these comments, imagine a button that said vote on steem, vote on eos at the same time, but the comment only exists on one.

if you are talking witnesses well same thing, how do you know the one there and the one here are the "same" one?