We have a double at the mo, and wanted more room, and we both like different mattresses! So I've trawled the shops and internet on this subject. What we finally bought (today!) are zip and link beds. Two 3ft singles Divans with mattresses that zip together. So one side can have a softer mattresses than the other. So it can either be a super king size or two single beds. Hope this makes sense.
We have 2 single bed (seperate bedstead and mattresses) joined to form a ?Superking, I think (I get confused with Kings and Superkings: total size is 2x2 meters.
Works really well IMO, we have been sleeping like this for 7 years: loads of space (well, not so much when little people join us...), we have different hardness of mattress to suit DH and me and in a previous room in a previous house these were single beds, so could be again.