What is the source for having separate mattresses while niddah? What does a couple do if they are traveling and there is only one bed available for them to sleep in? Can they place a partition between them and share the same bed?

Shalom The source is Shulchan Aruch Yoreh Deah 195:6. The should look for another solution such as one of them sleeping on the floor. Some are lenient to place a significant barrier between them if it is only a very occasional basis. Kol tuv