You can only use up to two Heroes at any time but you can choose which to take into battle before it begins.
Select the Heroes who will the most appropriate for the Level you are tackling!
When selecting a level, you will be prompted to select Heroes to take into battle. If you have both Heroes selected, you can switch them out by tapping on the Heroes pictures.
I you only have one Hero selected, you can add another by tapping on the "+" button.
You can only select Heroes that you own.