Use iPhone and share audio to another device

Hi. Hoping someone has a solution to a somewhat unique problem.

We’re trying to use one iPhone to talk on and listen to the call while sending the audio of that call to another device for a second person to only listen to (no mic).

Can anyone think of a solution that can make this work?

Pretty sure we both have lightning connections in our iPhones.

We’ve considered using a lightning splitter with two 3.5mm jacks, but those only seem to work with audio only. I haven’t seen one that allows one set of headphones with a mic and one without.

Any ideas? Thanks.