I'm sorry, but it is really simple, the main thing to remember is that the ships are practically identical. Pkus both ships alternate itineraries, but one ship is going east while the other is going west.
The ports of call are the same for both ships during those alternating itineraries.
Does that make sense?
Oasis sails the eastern caribbean one week, and then does the western Caribbean the next week.
During the same two weeks: Allure will go to the Western Caribbean the first week and then go the the eastern Caribbean the second week.
The ports of call, during either the eastern or Western Caribbean cruise, by either ship, are the same. So essentially the experience is identical on either ship. It is just up to you to choose which week you want to travel and/or where you want to go.
If you want to go to the Eastern Caribbean on a certain date you will choose the ship that is going there that week. If you want to do both cruises back to back you stay on the same ship. It is just a matter of choosing which date you want to sail and whether you want to go east or west first.