The longer answer is that as a leader, it's your responsibility to maintain an atmosphere where your people can be their best. Allowing one staffer to create a rift in a professional relationship doesn't occur in isolation. It's a conflict that affects and stresses everyone around you. Unless you protect the environment for your people to focus on productivity, their performance will likely worsen, or they'll leave for a workplace where such squabbling isn't condoned by management.

To alleviate this, you should first meet privately with him and ask for an explanation of his behavior. Be prepared for him to outright deny the fact of the ongoing tension, or to even blame the other person.

However he responds, the next step in this sequence is to furnish a few clear-cut examples of the behavior. Then, state unequivocally that the friction cannot go on. At this point, give him an either/or choice. Offer him the chance to dial down the tension immediately, or propose that you run interference for him if he feels he can't untangle the situation himself. By aiding him in making peace, you'll also help the entire office avoid getting caught in the crossfire.E