How does the XP system work in co-op anyway? I would guess that only players who participate in combat get XP for slain enemies. If a co-op player enters a combat of their partner he should receive XP only from that point onwards. Is it like that?
And of course the second player out of combat shouldn't be able to hurt enemies in real time. Maybe it would be a solution to make the turn-based area much bigger but giving the second player double the action point in the first turn when entering the fight. That way he could run to help his co-op partner in turn-based mode, but without the change to do damage in real time.
I don't now about the XP even though I played the whole game in coop, because the TP Pyramids allowed the other played to join you very quickly.
As for the entering combat issue: This seems fairly straight forward. Figure out what the absolute maximum range is that someone could cast a spell to influence a combatant in the optimal case. Make a radius of that range +2 or so around every combatant an area that triggers combat for everyone that is not in combat, regardless of what they are currently doing.
As for more actions point when someone joins combat late: I don't see why time should run faster for a character just because he is further away from combat. A turn in combat is 6 seconds (I think) and your AP represent what you can do in those 6 seconds not more, not less (sans buffs/debuffs).
