You lost me there. You would have two belt pack transmitters, one on each interviewer/ee. Then two belt pack receivers plugged into your recorder. Assuming nothing for ambient or boom mic or whatever, I'd just plug one into one side of the recorder and one into the other and mix them in post.
Yes...I got carried away with belt packs:
Interviewee belt pack transmitter /lav
Interviewer carries receiver .. Interviewers mic directly in the recorder and receivers output into side 2 of the recorder...
This is NPR - with just the 2 people unencumbered by cables or anything , people with booms etc.
This is what she is looking for....so now it is down to picking the best option among batery powered transmitter / receiver..