In these cases you normally don't use stereo samples, especially if you use MIDI or a Tracker to play back the music. You just set a stereo panning for each instrument/effect, so when it gets mixed into the output signal(s) you get the positioning. If you really, really need a pre-mixed stereo effect it would be stored as two separate samples that are started at the same time. (Though I haven't checked the datasheet if the sound chip has a built-in stereo sample format.) It DOES support 16 bit samples, though that seems to require DRAM for the sample data.
It would be interesting to see if any games have a stereo mix, it may be possible that they prepared it in case a future motherboard has stereo output. I think someone on the PGM conversion thread posted a picture of a PGM motherboard with the MVS stereo mod PCB without the middle channel connected.
Apparently many PGM games use a Tracker based music player, so if the driver supports stereo mixing it may be possible to patch games for stereo support.
Edit: Linky:
https://vgmrips.net/forum/viewtopic.php?f=15&t=2231
I've seen it first on the PGM Conversion thread, but right now can't find the post.
Edit: So the MOD version of the Espgaluda soundtrack definitely has panning/stereo, though someone would have to check if the driver actually handles this. The XM doesn't, but that seems to be a limitation of the file format. Renoise (
https://www.renoise.com/ ) can't open the MOD files, but it can open the XM files (without stereo). I managed to use OpenMPT (
https://openmpt.org/download ) to convert the MOD into IT (Impulse Tracker), and open those in Renoise with stereo.
