How to play local couch co-op on Balder's Gate 3 (Steam)

 It was not intuitive how to play local couch coop. I hope this helps someone out there.

Step 1: Open Steam in "Big Picture Mode" (this loads the drivers for all connected inputs like xbox USB controllers)

Step 2: Buy Baldur's Gate 3 for the account that will use this.

Step 3: install on a PC that meets the minimum specs

Step 4: verify that both controllers (not a keyboard and mouse) correctly work with Steam Big picture mode by using the joystick from each to move the selected button THEN launch Baldur's Gate 3

Step 5: briefly used a keyboard to get through the larian launcher. You should now see a splash screen like this:

Step 6: press a key from one xbox controller to switch from keyboard input to xbox, then press a key from the other xbox controller.

Step 7: voila. Rock on.