The verandahs are divided by solid partitions, but they can be opened (by the crew) if you have adjoining staterooms. So, for example, staterooms 12006 (1 bedroom suite) and 12008 (verandah family stateroom) have a connecting door, and their verandah divider can be opened to create a single large verandah as well. (See next post for photo.)
For Caribbean/Bahamian cruises, I prefer starboard for 2 very specific reasons: (1) you get a really nice view of Castaway Cay from your verandah and (2) you can watch the fireworks from your verandah on pirate night. Beyond that, port vs starboard really is just personal choice. For no particular reason, we had starboard staterooms for our first 4 cruises. This last cruise, we stayed on port side and it felt a little disorienting at first, but I didn't notice any difference beyond that. (As you may know, deck 11 only has rooms on the port side.)
We prefer deck 12 because of the proximity to the Concierge Lounge, because we spend a lot of time in there. I didn't notice any noise from the sundeck on deck 13, which is adults-only.
Thank you so much for your response! This is really helpful. We are going on the Fantasy June 2016 with 15 people. This helped make our decision to move to concierge. Thanks!