Make sure you have at least 2 players mapped, then when in the Join Game screen only join one of them.

When spawning in your character, make sure you're spawning in the player number you aren't playing as. For example, if you have 2 players mapped (players 1 and 2) and player 1 is the player that joined, spawn in with number 2. To spawn, point your mouse where you want to play as your character, press CTRL+<Player Number> and that's it. If you dont want to spawn a boss do SHIFT+<Player Number> instead. For example, if you spawn in the doorway of Ring you can play on the bleachers and even fight there as well.