I just did a test by copying a mono test audio file over 4 channels (hard panned left and right alternating) in Audacity and exporting a multi-channel FLAC file. The result in XMPlay is the file info shows 4 channels and while I currently don't have multi-channel hardware to test, there is audible difference depending on what settings you choose in XMPlay:
When using MM100 or DirectSound output, the two latter channels appear quieter and panned more to the center. I suspect this is Windows simulating rear speakers on a stereo device. When enabling "Downmix multi-channel", the panning and volume of the "rear" channels change to be identical to the "front" ones.
Unsurprisingly, downmixed channels sound the same through WASAPI output also but if downmixing is disabled, channels 3 and 4 disappear entirely (from my stereo headphones). It seems to me that this would be the desired option when rocking an actual surround setup, bypassing any simulation Windows might be doing.My advice would be to convert/mux the two stereo files into one multi-channel file to save you from the trouble of trying to sync two independent players every time you want to play the track.
This way you'll also have the flexibility of either downmixing, simulating or using actual multi-channel when your sw/hw environment or audio setup changes without having to touch the file(s).
If you need help with that, I now have some experience.