Agaricus bisporus
In the rather bizarre situation described I'd firstly try to talk the customer out of it,
Why..... we have clients who want to do this all the time. They feel more comfortable having two people looking for traffic etc. We are flying them low level through the Los Angeles Basin. We have two fully qualified pilots, both certificated in the aircraft. One does the flying, the other does the radios and call outs etc. On multi-leg flights we switch up who is primary each leg. All VFR.
It is not a problem as long as we do a briefing prior to going.