Greg, I have the same 2 channel S/PDIF optical problem as you do. It’s been such a headache I just gave up and stayed with 5.1 analogue connections for now.
If you do an internet search for something like “only getting stereo from sound card with S/PDIF” or some other similar wording you will find this is a common problem. In theory all you should need to do is set your sound device and playback software to pass thru S/PDIF, AC3/DTS or however your software lables it. However, it didn't prove that simple for me.
Wish I could be more help,
Dean
