Best thing to do beforehand is to arrange for somebody to accompany you underwater to spot for you or who just wants to take it easy or preferably is another photographer.
On my recent trip to PG, there were generally around 4-6 divers per dive and all of us were taking photographs, I did manage to lose the main group a few times but it was never an issue as I eventually found them again at some point.
Two photographers is realy just 2 solo divers.
Having my buddy is the best solution however she does not want to dive as often as I do.
A dive guide is another option I have used before, particularly on night dives. However you have to seperate from the crowds otherwise you end up just being mobbed.