One double mattress or two single mattresses?

Both work. With one 160 or 180 cm mattress you sleep without a seam in the middle, and you can still choose a different firmness for each side, with a maximum of one firmness level difference. With two single mattresses of 80 or 90 cm you have complete freedom: firmness, materials and comfort can all differ. Do choose two mattresses of the same height. A topper over both mattresses turns them into one surface again.