Is it better to choose one 180x210 cm mattress or two separate 90x210 cm mattresses?

That depends entirely on the two people sleeping in it. A single mattress gives a seamless surface with no gap in the middle. Two separate mattresses let each person choose their own firmness and material independently, which matters when two people have significantly different body weights or sleep preferences. With two separate mattresses, a Tuur® topper in 180x210 cm bridges the gap between them, so the surface feels like one uninterrupted mattress while both sleepers keep their individually tuned support underneath.