Do bring this up with the other couple. This is one of the hurdles I had to jump with Alex & Jen. I don't intend to live in this community forever, and I hope to get a two-year teaching commitment in Japan myself. Due to my career needs, I may not live in this town ever again. Alex and Jenny were both hesitant, knowing this, to really get emotionally involved with me, but we talked it out and came to the conclusion that if I do fall deeply in love with them during the time that I'm here, I won't stop loving them just because I'm far away.
The other couple may see it the same way. If you and your fiancee are going to be coming back anyway, this could facilitate things. And who knows? Maybe it'll be an incentive or an opportunity for the other couple to take that once-in-a-lifetime vacation to come visit you when you're living there.