well, when you summon the other form of yourself it turns on "herosummoned" switch
inside every monster there is a place for events called battle events.
trigger it by a switch, that switch will be herosummoned
add exp to the other hero and turn the herosummoned switch off
since your hero won't be back anyways you won't need to control him not being able to summon.
this will add the exp to him, as for the "summon" make it level 99 so it won't level up or give him the normal exp so it will level up whenever it's in battle
lastly outside the battle where the para process is add the hero back to the party and remove the "Summon"
hope i helped