When we took my twins on a cruise (at age 4), we brought umbrella strollers and stroller hooks. Stroller hooks will connect two single strollers to make a double. They have to be the same stroller model to work though. On the ship my kids actually pushed their own strollers. My son would have done this at 18 months too, actually, but you could have one adult for each stroller on the ship. Off the ship I hooked the strollers together. For the most part we didn't do excursions with the kids. I just walked around the touristy port areas with them so I needed something that would work as a double for when I had them by myself. The stroller hooks make an extra wide double so they only go in very wide doorways when connected, but they are pretty easy to unhook and rehook so I just pushed one kid through at a time when necessary.
Another option might be a TwinGo carrier for you, assuming you don't also have any back problems, although if your kids are on the larger side that might be a lot for you to carry at once.