Welcome to the forum!
S/PDIF can indeed play multi-channel. The best way, I believe, is to set up your computer to send the raw digital bitstream out the digital audio output. If you are sending it as PCM, then it will be two-channel (as far as I can remember). On my computer, I had to set the sound card to use the digital output (it couldn't use both analog and digital at the same time), and I had to set up my software program(s) to send the straight digital stream. Your computer may be different, but I wanted to mention that because you might have to set that in more than one place.