Just played a game where I had a heck of a time getting armies to stack using Kraxis, particularly when I had a Kingdom hero and an Empire hero. At first I couldn't get the kingdom hero to stack with the empire hero, but then I got them to stack by having the K hero go solo onto the tile with the E hero. I got a pioneer to join the army, but then I couldn't get another militia troop to add to the stack. I couldn't find any rhyme or reason to who got left out. Sometimes it was a hero, sometimes it was a troop unit.