Hi pmsmith2032.
Regarding selecting a mattress, I’ll refer you back to my earlier replies to your previous thread, specifically posts #2, #4, and #6 (where I also answer your question about Brooklyn Bedding).
If you’re considering using two twin extra long mattresses, and if they are on separate bed frames and separate foundations, that is certainly the best way to minimize motion transfer, as you’ll have two independent units. If you’re placing two twin extra long bed sets upon the same frame, you’ll still have the capacity for some transfer of motion.
In general, there is more about motion transfer in post #18 here. Memory foam is the best at motion transfer but latex, pocket coils, microcoils, buckling column gel, and even polyfoam are generally good as well but it will depend to some degree on the specific design of the mattress, on your relative weight ranges, and your sleeping style (such as how close you sleep together). Pocket coils are generally good for motion transfer unlike innersprings that are linked with helicals which are generally poor to fair depending on the specifics of the innerspring and the foam layers above it. Just as an example most people will tell you that a mattress that uses latex comfort layers such as the BME would be relatively good at motion isolation but there are also some that will tell you otherwise which is a good example of why once again the most reliable way to know whether a mattress is “motion isolating enough” for you would be based on your own careful testing or your own personal experience with both of you on the mattress in the positions you normally sleep in.
Good luck!
Phoenix