Nuh nuh nuh no.... !
You have to stay on the same ship or else you will be doing the same itinerary again. Both ships alternate itineraries on opposite weeks. They do it on purpose so you can do back to backs.
They are so close you will enjoy Oasis as much as Allure.
I mean - unless you really don't care about ports. I just hope you get to see Labadee because the zip line is very cool.