I have 2 sound cards in my PC running PlayoutOne Standard.
I can only see Input 2 linking to input 1 or for the mic input, but nothing to let me link it to my second sound card in order to use it with another fader, I did download Virtual Cable A B C & D thinking that would cure it, but Stereo Tool input 2 is, as I mentioned, not letting me do that.
I'm still not really clear here. What are you using for your "Input #2 Usage"? What exactly are you trying to do?
I/O > Soundcards > Input #2 > Input #2 Usage > Use for: Combine with input1 (add) should do what you're asking if I understand correctly... and I don't know that I do.