A couple of things to consider if doing the 2 Caribbean island trip:
1. Even though islands may be close (even very close) geographically, some flights will go from one island to another via Miami, San Juan & JFK, or Charlotte. It can make for a long day, and in some cases, these flights involve an overnight layover.
2. Some islands have short puddle jumper flights -some expensive, IMO, and some not too expensive - ferries may be an alternative but can't be found just anywhere. St. Maarten to Saba has both, but the low flying, 12 minute flight has shot up in price from a few years ago. The ABC islands are close to each other, but don't have ferries (to my knowledge) and flights don't just go from one island to the other.
3. Some flights/ferries only travel on specific days and at specific times.
My one "2-island in one trip Caribbean experience" was flying from Puerto Rico to Grand Cayman via Miami. Only dove 1 day during the week in PR as we did not stay on the best side of island for diving - was glad to have spent 2nd week in GC diving everyday, but we lost a day by flying. On the plus side, I didn't have to pack wet gear to lug around to, in, and from the airports. Wouldn't recommend it for shorter vacations and I don't see myself doing it again unless it's super convenient and not too expensive!
Here are a couple of suggestions, whether you island hop or stay, that fit what you're looking for. For an 8-10 day stay, there is enough to keep busy whether it be diving or topside for the activities you want.
Grand Cayman is a nice dive location with short flights to sister islands. Can dive different sides of the island, shore dive, and hit a couple of wrecks. Some say the diving on Little Cayman is better.
St. Croix has some varied diving - reefs, wrecks, walls. There aren't a lot of cruise ships coming in during any month which is good as the pier is a highly popular and recommended shore dive (day and night.)
* Both GC and St. X have a calmer west side with good diving if other sides of the island become a little rough. Maybe an important consideration when vacation days are short in number.