You can't have two islands in one mission.
However, you can create the illusion of it in a campaign. That's what BIS did. What appears to be one mission is in fact two missions, one following seamlessly from the other. The second mission has no Briefing to get in the way (or the Briefing is skipped), and there are no Intros or Outros involved either.
The way a campaign "works" is handled in description.ext. There are tutes on this in the Ed Depot.
There is nothing wrong with creating a Campaignette which has "one mission" set on two islands. In fact, the more I think about it, the more I like the idea.