Hi Pained_notPoor,
The best way to make sure that a mattress doesn’t have too much “roll together” in the middle is based on your own careful testing (hopefully using the testing guidelines in the tutorial) with both of you on the mattress in all the sleeping positions that you normally sleep in but there is also more information in the first part of post #2 here about some of the different ways or mattress designs that can be used to accommodate a couple that have very different body types or different needs and preferences in a mattress.
Phoenix