I suspect this would be rather difficult to achieve, you’d have to have the right situation for sure. Would have to be an apartment or flatmate arrangement.
I think a recording of a roommate “joining in” as he / she listens to his / her roommate having sex would be hot - if it were recorded by one or both members of the couple being overheard. “Voyeuring the voyeur.”
For example, if you know or suspect your roommate in the room next to yours listens to you having sex and masturbates, you could place the recorder outside his / her door and capture your roommate’s sounds of pleasure as he/she listens to your lovemaking, with you and your lover’s sex sounds there, faintly, in the background.
I think that would be even more exciting than one of my favorite type if recordings, the recordings where the recorder captures him or herself “joining in” with the couple he or she is listening to, gets so excited he or she clearly climaxes, but the focus of the recording throughout remains on the couple having sex.
I wish I’d thought about all this stuff and this forum had been around in the ‘90s … the sounds I could have brought you!