Go on adventures together then make a dungeon for each other with the stuff you unlocked!
Co-op Adventure
Work together to overcome increasingly difficult dungeons. The rewards are new dungeon building blocks to use in PvP Duel, as well as new Heroes! If players have different progress, the one with more progress will help the other up to his level before they can both progress. PvP ~ Build
Make a dungeon for the other player. You have a wide selection of building blocks, such as Enemies, Npcs, Traps, Loot, even Triggers and Events. You can also change dungeon settings such as Theme, Preset and Objective. PvP ~ Test
Before the Duel begins, you must successfully test your dungeon so the other player isn't stuck in something impossible. If a hero dies or you delete them you go back to building. You can test and edit your dungeon as many times as you want. PvP ~ Duel
When both players successfully tested their dungeons, the Duel begins! You will each play the dungeon the other player made.
Thanks to phdorogers4 and Zenophobia for playtesting!