I would say it all depends on how long you are in Grand Cayman, how much time in between the 2 tours?? Remember also, you have to tender there, and if you are doing a ships excursion, you will be able to tender sooner than the rest. I wouldn't cut it too close for the 2nd one??
Carnival has a Stingray city and island tour. I would think that the other lines would have something similiar. You hit the highlights of the island prior to going to stingray city (Hell, the turtle farm, ect).
Just got off the Carnival Imagination, which stopped in Grand Cayman. We docked at 7. We were able to do two excursions but the time was cut kind of close. Our first excursion was the Sea and See Tour at 8:15. We didn't even get off the boat until about 9 and the tour started around 9:15. I'm not sure why they did it that way. Anyway that tour was a lot of fun-- we did a glass bottom boat, went to hell, the turtle farm, stopped at the beach, and ended in Georgetown. Then we had booked the 12:45 tour of Stingray City. We made it back to Georgetown around 12, walked around and grabbed something to eat then made it back to catch the bus to the boat. Stingray City was interesting-- I don't like swimming with the fish but it was interesting to watch. Everyone in the water is screaming and jumping around! The stingrays are huge! We made it back in time for the last tender back to the ship. So we had a real busy day and saw a lot.