The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Sav, addresses the question of whether a husband and wife may sit or sleep on each other’s bed during the period when the wife is a Nidda (listen to audio recording for precise citation). He writes that a husband may not sit or sleep on his wife’s bed during this period, even if she is not present, though he is allowed to touch the wife’s bed without sitting or lying on it. The wife, too, may not lie down on her husband’s bed, but she is permitted to sit on his bed during the period of Nidda.
The Ben Ish Hai also addresses the question of whether the husband and wife may sit together on a bench or sofa. The Bet Yosef records a practice observed by the Ashkenazim to refrain from sitting together on a bench that jostles when somebody sits or moves on it. According to this custom, when a wife is a Nidda, she and her husband may not sit together on a bench if they can feel each other move on the bench. The Ben Ish Hai rules that Sepharadim should also follow this custom, noting that this was the prevalent practice in Baghdad. Hacham Ovadia Yosef, however, disagrees and rules that Sepharadim are not bound by this stringency. According to all opinions, it is permissible for the couple during the period of Nidda to sit together on a heavy sofa, which does not move when a person sits down, moves or stands up, provided, of course, that they do not touch one another.
If a husband and wife sit together in the back seat of a car when the wife is a Nidda, they should put the wife’s pocketbook or some other object in between them to ensure that they do not touch each other when the car turns. It goes without saying that they may sit side-by-side in bucket seats in a car.
Summary: When a wife is a Nidda, she and her husband may not lie down on each other’s bed, and the husband may not even sit on the wife’s bed. According to some opinions, they should also not sit together on a bench that moves when somebody sits down or moves on it.
