So heres the info you need. And unfortunately its giving me problems but itll be fine for you.

With 1 steam controller, I've noticed its set to player 1 by default. Even if you plug your other controllers in first, the steam controller takes precedence and becomes player 1. If you want to do 2 player splitscreen you need multiple controllers. So get your 360 controller and plug it in and your steam controller will be player 1 and the 360 controller will be player 2. If your controller is wireless you need to go and buy a Bluetooth adapter for your PC (one of those usb sticks is fine or get a PCI card)

My problem is that I have controllers but I want the steam controller to be player 2 because its wireless and my other controllers are not. I'm still trying to figure out a way to get it to be player 2 with my ps3 controller to be player 1. I might just end up dropping another $60 to buy a second controller anyways, that way I have 4 controllers (2 steam 2 ps3)

Im having the same problem with Portal 2, I want to be able to re-assign the steam controller as player 2 so that I can play split screen using my mouse + keyboard as one player and my steam controller as the other. Is there any way to do this?
I tried the launch options and console commands to re-assign the joypad to player 2 but this didn't work because I think the steam controller isn't detected as a standard gamepad.

Any help would be greatly appreciated as I dont want to have to buy or play using an xbox controller.