Here in Germany it has always been the norm to have two mattresses, each with their own sheet, and each partner has their own duvet, too. That way, you can have a mattress and a duvet to suit you. No one is really bothered by the "visitor's slit" (doesn't sound so naughty in German!). In fact, you often have two beds pushed together so you have the frame of the bed in there, too*. Nowadays, smaller double beds with single mattresses are becoming available, but the Germans are used to having loads of space and certainly don't "get" the idea of a single duvet for 2 people.
We have always had a single, large mattress, (and a king size duvet) but we've just recently bought new mattresses and the adjustable base to go underneath them. They were inordinately expensive, and I don't think my husband has ever used his to raise his legs or to sit up. I use it to watch films on my laptop in bed, and always raise both ends to take off and put on the sheets when I change them. You press the button to lower the ends and hey presto! the bed is made! The salesman demonstrated to us how relaxing it is to lay on the bed and raise your top half and then slowly lower it. By raising and then slower lowering the leg end, you can relieve a troubled or overfed tummy.
I had to buy new fitted sheets - the single sheets we had up to then for the kids' beds were not high enough for the 26 cm deep mattresses!
I am still not convinced it was worth going to a specialist shop for all this kit. In the end, it's what you're comfortable on, and that could be a cheap mattress on a bit of chipboard for some.
* Don't ask me how they cuddle up!