Gregory's Server@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.
@d6 Another thing to consider in your divide step is that you’re introducing aharmonic quantization error. If you want to get fancy, add one bit of noise to your 16-bit sum before dividing.
This will cause your error to be white noise instead, which sounds a bit better.