My ideal multi headed P-60 would be silimar to the Klarus XT20 or the Shadow TC-750X. Even better would be side by side 18650's with two switches on the head one for each dropin. Then the sky would be the limit on dropin combo's. Current would have to shared by both batteries regardless of which side was on or both being on.
From a ergonomics standpoint a 3 18650 parallel setup would be better as the triangle shape would be more round in the end. A 2 parallel setup could be made round as well, but that would be a waste of space in my opinion.
I know everyone has their own opinion but so far I'm really digging a triple P60 host that has a head resembling a Fenix TK45 and a body/battery tube like a Thrunite TM series.