@d6 averaging is the correct way to do this. The problem you’re experiencing is non-linear human perception. (And maybe some cancellation of out of phase frequencies)
You’ll just have to fudge in a volume boost… which will probably take you out of 8-bit range.
@d6 when thinking about the volume, also keep in mind that you’re going from two speakers at full volume to one speaker at full volume.
Try this: add the samples, don’t divide, and clip/saturate them instead. This will cause distortion, but it should be roughly the right loudness.