First, welcome to FT and congrats on the upcoming marriage.

The good news: you should be able to find lots of availability from CLT to the Caribbean in July. I just checked a few sample dates and see lots of availability to places like SXM, STT, and SJU.

The bad news: mid-July will be HOT down there, which is why there's tons of award availability.

To get two first class tickets down there, you're looking at using most of your UA miles and then you need to increase your US balance to 60k. If you don't have the USAir credit card, that would be an easy 25k.

Or, if you don't mind making a connection in MIA, you could get two of the AA credit cards that give you 75k miles each. With the 150k from that, you'd be able to get 2 award tickets, although you might have to fly coach since the CLT-MIA flights are all coach (I think) and it might not be worth it to pay double the miles just to sit up front for the short MIA - Caribbean flight. Or, both of you can get two AA cards and then you can literally get almost anywhere in the world in business or first class.

Another thing to think about if you end up on US from CLT to the Caribbean is what type of plane you're on. For example, US has a few flights a day from CLT to SJU, but some of those are operated with an A320 and some with a 757. The 757 will have a much better seat than the A320. The difference between the F seat on an A320 and the F seat on a 757 is greater than the difference between a coach seat on either and an F seat on an A320 (IMO).

Edited to add:
Caribbean flights in the summer are fairly easy to get elite upgrades on. Since flight costs seem rather reasonable (~$400-$500/ticket), it might be worth it to just buy the tickets on USAir, then sign up for the trial preferred with USAir (so that your honeymoon falls inside the 90 days you'll have status), and hope for an upgrade. You'd be out ~$1000-$1200 and might not get upgraded, but this is an option that will let you save 120k miles.