If you use one sound card then this ain't gonna work.
The signal path thru the card is common for all inputs even tho volume control can set up levels and mutes for various inputs - signals are mixed. Also the output is the same going to headphone skt as well as line out which you'd usually use to feed your speakers, with their own amp.
I have never tried more than one sound card but unless I am forgetting something - I think you'd need to have another to do what you want.
