Hi Kate.

Memory foam mattresses are notoriously hard to ziplink as the fabric used in many cases is a stretch soft knit rather than damask as found on firmer mattresses.

With this in mind, your situation requires an added layer of difficulty by asking for two completely different kinds of mattress but at the same level to fit your frame.

This request is fraught with complication and unlikely one that many manufacturers would be happy to carry out at an acceptable price.

A posible solution would be two separate single mattresses (but special size 6'6 long) joined together by use of good quality protector.

This is where bespoke manufacturers come into their own and more specialised than most in being able to help in situations like these.

Try contacting Burgess Beds directly and ask them for a suggestion.

Please keep me updated with your progress as this is a much asked request.

John.