This was our main question from our first play-through...
Captured leaders automatically oppose missions against them, ie. Lure of the Dark Side, but can you also send another character to oppose the mission as well? It doesn't make much sense that you're interrogating a captured general at some black site and his pal suddenly appears for moral support.
Yes, one can send a leader to oppose. RR, p. 09:
When a mission is attempted “against a leader,” that leader automatically participates in the mission.
–– All leaders in the system participate in this mission.
–– If a player sends a leader from his leader pool to oppose this mission, the mission is still being attempted against the original leader.
As with a lot of these missions, you have to get a little creative to get a thematic reason for a leader to be able to oppose it. Instead of thinking that someone is showing up for moral support imagine, say, that the Rebels find out what's about to happen and send an operative to intercept the Interrogator before he can get to the captured leader to use the probe droid on him.