First, it’s important to let the two employees resolve their conflicts without interference from Human Resources. But, if it becomes clear that unsupervised resolution isn’t likely, mediation is your next best step.
In mediation the needs of both employees should be acknowledged. Emotions make for difficult terrain in a workplace, so it’s best to empathize with all parties, making it clear that you will not take sides.
Once each person feels heard, there is a possibility they will be able to think more clearly about a solution.
Sometimes when we are angry we tend to be fatalistic and believe there is no possible way to resolve a conflict. Getting both employees to calm down and sees things with less emotion and more logic is the best first step to agreeing on a solution.