If a co-op player enters a combat of their partner he should receive XP only from that point onwards. Is it like that?
I highly doubt it. That would be a programming nightmare. I mean I guess they could run a variable for how long the combat lasts and devide how many turns the second person participated, but again, that would be annoying and lead to all kinds of bugs.
I don't think it's that hard to program tbh. The system already notices whether a player is in combat or not (it determines whether the player is in real time or turn-based mode). In that case the second player not engaged in combat just wouldn't receive any XP points from his partner's kills in combat as long as he is in real time mode. It's a simple check: real time? yes -> gets NO XP, no real time (turn-based)? -> gets XP
I don't know why this should lead to many bugs...
