Hi ritaelyn,
You’ve already touched upon some of the pros/cons of split layers, but I go over these in much more detail in post #2 here, which you may find interesting.
If you had a concern, you may wish to go with the upper layer as a solid sheet (with your sleeping in the center), but in most instances, unless there is a larger discrepancy between the comfort of the layers on each side of the mattress, most people do not feel the area in the center of the mattress where the layers meet unless they are quite sensitive. A more substantial mattress encasement also assists with minimizing the ability to feel the center area where the layers meet.
Overall, and this isn’t to minimize the comfort of your boyfriend 
, I would recommend that you choose a configuration that works best for you, as it is your personal mattress. But having the layers split does allow for more flexibility as you mentioned.
Phoenix