Just have them in 2 seperate parties (not a raidgroup).

Then, as you steer from your main, have 1 button that makes only members in party nr 2 tags mobs and gets credit. And 1 button for those in party nr 1. I try to time it so that when everyone is "9 / 10" complete, i finish off party nr 2, then party nr 1. Then carry on to complete the next quest.

The addon named "questplates" is very handy.