While that would clearly be an uncomfortable situation for Student B, I'm not sure how that is different than when someone accuses their non-dorm neighbor of rape. Is it reasonable outside of school to have to live next door to your rapist while the police and courts slowly do their job? While the hope would be that if the evidence is clear, the suspect would quickly be taken into custody, obviously this doesn't always happen. And obviously, sometimes the evidence isn't clear.
Outside of universities, we've decided on a particular balance between the rights of the accused and the psychological well-being of the accuser. Perhaps we should change the system everywhere to more greatly favor the accuser and to lower the standard for guilt in cases of sexual assault, but I don't see why universities should have a different standard than the rest of society.