I've done back-to-back excursions in a port pretty much without any issue, even with as small of a window as an hour to hour and a half in between. I only once had a Princess excursion that went way over the time because the guide wanted to take the group to another place that wasn't on the original itinerary. We arrived back after our other excursion left, but then went and explained the situation at the Shore Excursion desk, and they provided a full refund for our afternoon tour. If you can, I would book the tour you care about most as your early one so just in the rare instance it is delayed you don't miss out on what you really wanted to see and do in that port.