Arrange their working conditions so they no longer have to work with each other or be around each other, if possible.
If not, have them each air their grievance with you personally. After the meeting with each of them, say you will look in to reprimanding the other one. Then do nothing.
Another possible solution would be for them to file written grievances with HR. That will allow you to do pretty much anything as far as moving one of them to a different unit, or changing their job so they do not have to work together.