I don't think synergy of Champ with Champ is a good idea. Afterall, champs already get an XP penalty for joining up in the same group. To make matters worse, you only realize this fact AFTER the battle is done and XP is divided up. There should be some warning or icon denoting the limted XP that your champs are getting.
What I do think is a good idea is to have some Champ types grant additional tatic bonus to differnet types of troops that are grouped with them. A type of 'leadership' bonus if you will.
Assassin grants bonus crit chance
Warrior grants bonus hp
Administrator grants bonus XP
Mage grants bonus magic resistance
This way, your troops can gain a different and unique perspective on any given battle, depending on who they're assigned to follow orders from.
Naturally, having multiple champs in the group would grant multiple bonuses, but the penalty percentage in XP would also be multiplied into those tatic bonuses. This would naturally balance the effect.
example: If the Warrior grants +5 hp per unit, then having another champ in the party would half that number to +2.5 hp per unit. Having 2 warriors in the group would still result in that +5 hp bonus since 2.5 + 2.5 = 5
Obviously this idea complicates combat even further...and would result in more programming, game balancing, and ideas...but it's an additional consideration to keep the player at guessing as to what the best combinations of forces there are....if EVEN there is a best combination...