As with many things, identifying the root cause or causes of the friction is a great place to start.

• Is it the less than optimal reaction of one employee that causes angst with the other?
• Is the apparently innocent one also guilty of initiating or tempting conflicts?
• Does one or both just don't like each other?

These simple questions can unearth a lot of things in the relationship, and depending on how much trust you have built with them you, you will identify the problem, or problems, which needs to be solved. This could be clarity of roles and responsibilities, expectations of each other and/or the business (including the manager, you), or just plain just-dont-like-the-person at first sight.

While this as a rule requires substantial patience and active listening, there are situations where parties to the conflict are too emotional to separate the professional requirements of the job from their personal issues with eachother. In these sorts of cases, you should re-set their understanding of the primacy of business objectives and so they must find a way to work without adversely impacting the business, and then take corrective actions such as serving file notes and other such measures.

What you need really is great listening skills to identify the apparent and subtle sources of friction.