I know what you mean, youi cannot selct the inputs in Windows, my other N12 Interface does haves this option(also Yamaha/Steinberg driver but FW).
But…the problem here is not the interface but SKYPE obviously, see the video. And there is a solution for Wndows I read in the comments.
Another idea maybe is not to buy a secnd interface but a simple, cheap Behringer mixer or so.
Connect your mic and keyboard or whatever to that mixer and connect its ouputs to the URC 1/2.