I have done back to back excursions in a few ports. In one, Barbados specifically, I did a snorkel/beach excursion followed up by a cave excursion. I believe there was only 15-30 minutes in between. It turned out that both were run by the same vendor, and the NCL shorex staff communicated directly with the vendor on what I was doing both prior and via walkie-talkie that day.
I have had no problems with same day excursions with an hour in between. That is a safe time gap for me, also with the understanding that in rare circumstances a situation could arise where I would not make it in time for my next booking.
Communicating with the shorex staff is key. In a different situation, they advised fairly about a chance that I may not make it back in time from a tour in the BVIs to then make a catamaran excursion. I believe it was only 15 minutes in between. Basically, choose my priority or risk paying for both and not making it for the afternoon excursion. I ended up just booking the first excursion, which was my priority, and took my chances on getting on the 2nd tour last minute. It turned out that I made it back to the pier with extra time to get on the 2nd excursion, but it was a popular snorkel trip and was sold out at that point.
In your specific case, I would only book the snorkel excursion at this point. Then talk to the shorex staff on the ship and see if there is any realistic way to do both. In frequent instances, tour times are changed or canceled.
I'm Platinum and a shore excursion junkie fwiw.