A Switch only co-op mode will support two players locally. Specific details have not be provided, but presumably the second player will take direct control of one of the AI battle companions
So you think you can slay all the monsters and win eternal glory, is that it? Bad news, hero. In the Spiral Shards, we can only send in one hero at a time to fight the Ceaseless Dirge. At some point, a flaming blue fireball, mech mortar rocket, or giant spike of doom is going to hit you in the face, and you’re going to die. That’s when the fun begins.
When you die, you’ll leave behind an Echo of your hero on the battlefield. The next hero in line will be able to resurrect your Echo as an AI battle companion. Take turns continuing each other’s games, fighting and dying with friends until you have an entire army of Echoes by your side. Everyone dies. Heroes continue.