I am trying to start local co-op using a steam controller and nintendo switch pro controller, when prompted to press start neither controller seems to work, both are connected by USB and both are configured as 'generic' gamepads in steam, any and all help is appreciated

If you are mac I'm afraid I haven't found a way for a steam controller to play well with any other controller, even a second steam controller. If you are using smapi make sure the game is actually recieving your controller input. I would suggest testing with each controller individually.