Can't say for sure, but Heart Mattress makes their own mattresses so can make custom work (they already make mattresses with different firmness on each side). We are very happy with our mattress from them, and their prices are very competitive. More on reddit.com

I think finding a base that would accommodate this would be difficult also. Maybe just a normal mattress and partner uses a wedge pillow to elevate their head? More on reddit.com

Rolling together in a queen is most often caused by the bedframe and second most often caused by the foundation. It is actually rarely the mattresses fault. Maybe you’ve already done this, but before you spend a lot of money on a new bed, have you insured that your bedframe has legs in the center to support it, and you have a strong foundation under the mattress? The best way to check. This is have one if you lay on the floor looking under the bed at the foundation. Then have the second person get in the bed and move around if you see the bottom of that foundation sagging down or flexing at all, you know you have a problem with your frame. More on reddit.com

Seriously can’t remember a split queen, there is a reason they are hard to find. If you have to do a dual mattress a split king with two txl is the only way to go. It would be worth going without nightstands or a bedframe if you slept soundly. Alternatively, I would look into solid foam like Tempurpedic that absorbs motion if you have to stick with a queen. More on reddit.com