Should I have zippered mattresses

I am male, 64 years old, 6ft 2", weigh 20 stone and suffer from a stiff back (ie it only hurts to bend down). I prefer to sleep under a light weight cover in a room temperature of 19 degrees. I sleep with a CPAP machine, and I turn over quite frequently during the night, which disturbs my partner.
My partner is female, 66 years old, 5 ft 6", weighs 10 stone and sleeps like a log on any bed you can think of. Her only issue (apart from being bounced around when I turn over) is temperature - sometimes too hot, sometimes too cold! I am not expecting advice on the latter!
We have a slatted king size pine bed, with which we wish to retain. This bed has 2cm deep sides to retain the mattress(es) in place.
I have experienced memory foam..........and hate it. My partner has no preference.
I think two separate 2ft 6" mattresses would be best, but I am happy to be corrected.
What would your suggestion be?