Can a husband and wife who are in a state of Nidda, share a bunkbed. One on the top bed and one on the bottom?
It is permitted to share a bunkbed, one above and one below.
However, if the bunkbed is rickety, and the motions of one will be felt by the other, it might be preferable not to use it.
Sources: Although there is a prohibition of sitting on the same sofa, this is because of the closeness that sitting next to one another brings (which is why it is permitted where there is something in between the couple – Rema 195:5, Nekudas Hakesef and Aruch Hashulchan). See also Terumas Hadeshen (251) who writes that it is permitted to travel with one’s wife in a carriage (but writes that to go on a special outing/day trip may be prohibitied, again, because of the closeness/intimacy involved). See also Yevakesh Torah, Niddah 20. Therefore, a bunkbed, which implies no more intimacy than one bed next to another with a gap in between (or even less intimacy), it would be permitted for one to sleep above and the other below.
However, when one’s motions will be felt by the other, it may be preferable to avoid the bunkbed, in deference to poskim who write that this constitutes a problem (see Taz, 195:6). According to the strict halachah, however, it is clear that feeling one another’s motions is only a problem in a certain context (of possible closeness), and it is not clear that this is applicable to bunkbeds, where there is a clear and large gap between the two.