1. Diffuse.  Here’s what you say, “__, I really appreciated that you came to me first.  This has to be a misunderstanding.  I don’t think __ would ever intentionally do that.”
2. Buy some time.  Ask, “Has this been happening consistently?”  Probably not, but maybe.  Doesn’t really matter.  You’re buying time!
3. Think of a counter example.  If employee A thinks employee B isn’t pulling their weight; remind employee A of a time when B bailed them out.  This is where it will end 99% of the time.  Woops, misunderstanding … let’s move on.  Laugh, smile, and nudge employee A off the brink and pull them back into the team.  Unless …
4. Understand that there is truth in every complaint.  Evaluate the situation.  Is it serious?  Do you have a performance issue?  Can you tolerate mediocre employees?  To be safe, document the issue with HR.  If it is serious …
5. Bring up the issue with the other employee … yourself.  It’s not the responsibility of the teammates to reconcile major issues, that’s your job.  The severity of the situation determines the severity of the meeting.  Stop by the other employee’s desk for minor issues.  Let them know the situation and your assessment of it so that they can share their side.  If it’s major, meet in your office or head out for coffee.  If there’s potential for yelling or cursing, it’s major.  Now you’ve got the entire story.
6. Act.  Let employee A know you’ve done your due diligence.  They will appreciated it because you’ve listened.  Share the course of action, but be vague — especially if you’re reprimanding the other employee.  If the conflict is very serious and you need to fire somebody, do so promptly.  An inability to act decisively undermines your effectiveness.

Follow these six steps and you’ll avoid the territorial office chest-thumping that brings productivity to a stand-still.  Conflict happens, but it’s your job to keep it from impacting the business.