Help them learn to work out issues without your intervention. Sometimes one or more of your staff may experience friction with others. If they come tattling on one another to you, Listen to them carefully. If someone is not fulfilling his or her own responsibilities or is mistreating another employee, you'll need to step in and Resolve a Conflict at Work. But if you're satisfied it's only an issue of competition or a simple personality clash, urge them to settle it between themselves.
Talk to the other person, and upon verifying that it's a personality issue, simply let them both know that they aren't required to be friends, only to get along and get their work finished.
Tell them both you believe in their abilities to work and get along. Then leave them alone, but watch carefully. Don't interfere unless they bicker in front of customers. Put a stop to anything like that instantly.