Yes. This is the most common use for a twin XL mattress, as it allows each side of the king size bed to suit the specific needs of the sleeper. This is great for couples with different desires in a mattress. For instance, one side of a king size bed can have an innerspring twin XL mattress, while the other side can have a twin XL mattress made of memory foam. The main thing to keep in mind is the height (or depth) of each mattress. For a flush appearance, the two twin XL mattresses should be the same height.