Oddly I can perhaps relate. I have Phonak aids, and have always found that if I take the LEFT aid out, the volume going into the RIGHT aid seems noticeably softer. But if I take the RIGHT aid out, the LEFT aid volume is absolutely A-OK.
I wondered if it could be related to the “master” aid - the one which communicates with the other aid and controls the button presses for program change & volume for both aids simultaneously. Perhaps one of these aids is the “master” and that could impact the volume going into the aid if you remove one of them.
You’re not imagining this. It seems to be my situation, too.