Hello, I would like to preface this post by apologising for my noobishness and by assuring you all that I've already searched for a topic to answer my problem.
It's not a problem at all, everyone starts out that way.
Is the island that you want them to go to inhabited by friendly units, or any units at all for that matter? A very simple way of doing it if that is not the case is placing a trigger covering the island which is activated by whatever side the players play on (BLUFOR, OPFOR, Independent, Civilian or Anybody).
I'm not positive this will work, and it's certainly not an elegant and simple method (I'm not "amazing" at scripting myself), but there's always room for trial and error.
You could try putting one trigger which determines when one of the unit enters it and one that determines when the other one does, and so on. Then you could place one more trigger that determines when these two (or more) triggers have been activated, meaning the units have entered the zone, and then ends the mission when this condition is met.
Problem for me is I have this idea, but I can't tell you how to do it simply because I don't even know how.
If this is appropriate or the way others think it should be done then I think someone much more competent at this than I am will be able to tell you how to do it. Or they'll tell you an even better way, which is a lot more likely.
Hope that helps out a bit mate. Good luck getting this sorted.