When the wife is niddah, a couple may not sit together on a seat where the movement of one person can be felt by the other. This could apply to sitting together on a couch. But it is not the case for a sturdy sectional couch if they are on different sections. Therefore, a couple may sit on the same couch if it is sectional, or has separate pillows along the seating surface, as long as they are not sitting close enough to touch.
The coziness depends on what they are doing. If they are sitting with a group of people, there should be no problem. If they are home alone in front of the fireplace with romantic music, that is another story. Therefore, it may or may not be appropriate to sit together on a sectional couch, depending on the circumstances. The couple should use common sense in each situation.