I don't see an option in the menu and I've tried going about attaching multiple keyboards how do you play in multiplayer?
Thanks for reaching out!
The game comes with no default controls for 2nd player, so you have to bind them in the options > controls > edit P2 configuration menu. From there you should be able to map either keyboard keys or gamepad buttons how you see fit, either by hand, using one of the known presets or via the device setup wizard. I realize it's not very user-friendly though.
There is one issue I've noticed lately though, it seems that if you bind a keyboard key for P2's "start" command, it won't respond correctly on the ship selection screen (even though with a gamepad it works). You can work around this by having P2 join the game after P1 has launched and gameplay has started. P2 can customize equipement just the same with a simplified interface at the bottom of the screen.
I hope it helps!
EDIT: today I'll be rolling out a significant update of the game, and I squeezed a fix for this issue last minute. So watch out for the release of v0.51.0a, with this one you should not have to use the workaround anymore. Cheers!