As FFF says, there is nothing to stop you both logging time as P1, as long as total time between you both equals total duration of flight, i.e. 1 hour flight, Pilot 1 logs 40 mins, Pilot 2 logs 20 mins, etc, etc.
Multi-crew does not come into it as only one of you is P1 (unless you are safety pilot if flying under the FAA FARs but thats a different story!!!). Not sure how a 'club' can stipulate who logs P1 either!!!
I have done this several times with friends on a long flights where we have swapped over P1 mid-air for a break.
Julian