hmm, not sure if it's something that can be described... you just kinda have to find it yourself.
for my wife and me, it took much dying

eventually we settled with staggering our charms:
she charms and engages.
i search for a pet.
her pet dies she starts running the mob toward me and I engage with my new pet.
she goes out to find a new pet, repeat until mob is dead.
this way, only one of us is stuck with the xp penalty for having a pet and we alternate so the xp evens out.