As far as I know this is not possible unless the game would render the sound twice for each device separately and you would have the possibility to set it in game.
I'm using two sound cards too and I'm effectively splitting two sound streams to two devices but the sources are different. I'm able to play a movie with one sound stream on one device and play a game with different sound stream on second device.
As far as I know there is no way how to do split one stream to pieces on two devices. You can play the same thing on both, but you have no way how to identify what parts of the stream are what sounds. To do this, the game would have to support this but as I said, it would have to render the sound twice for each device separately which would mean more HW stress and it would probably be hard to code.
You should ask the devs to implement this in the game.