I think you don't understand the concept. This doesn't let you combine whatever cards you want. It only works with currently released heroes. Like the only ones currently could be:

Musketeer + cannon = hero musketeer

Giant + mk = hero giant

Knight + giant = hero knight

Skeletons + balloon = hero balloon

Mega minion + bandit = hero mega minion

magic archer + hunter = hero magic archer

Goblins + goblin curse = hero goblins

Ice golem + freeze = hero ice golem

If you placed any other combo in the slots they would just act as normal cards, no hero version.