All leaders present participate (exception: captured leaders only participate if they are targeted). So if the Rebels happened to have a leader already present in addition to the captured leader, and assigned another, all three would oppose. Likewise, any Imperial leaders that were there from earlier actions would help the Empire.
we did it all wrong... we only used one leader at a time.
the fact that all leaders present in a system participate in all missions changes a lot.
Thx for the clarification Doug!