Looks like you get to play marriage counselor. What you need to do is do what a good marriage counselor does.
You meet with each person individually, maybe a few times. Figure out what they feel are the pros/cons/issues in their relationship with the other. Focus on the pros and make them come up with these or even nudge them to some pros.
After you get their views down and form a plan you all three need to meet and lay it out on the table. Not getting along is one thing, but going behind each others back and bottling up issues in a small team is a cancer.
You need to act as a mediator. Let them know their status at the company and how important it is. And focus on the strengths of each. Let them also go over their grievances. Make sure they aren't calling each other names or something over the top but let them get it out.
This may take more than a few sessions and I might plan the first session with them as a couple hours and at the end of day. I have had to do this on a couple of occasions. I have only one rule with the people involved. Whatever they bring up in the room stays in the room.
If both of these employees bring something good to the table you can brainwash that stuff into their heads. If one of them has a master plan of hiring a pal or buddy to help you can get that our of their head. The goal isn't that everyone is holding hands, it is that there is a mutual trust. Run these sessions as many times as you need until you trust the issue is mostly solved.
Another approach I have used is to start out the counseling and if it starts going bad try to work in the worst possible person at the company I can feasibly get into the equation - I often ask for one of our general PMs. Make this person run some PM duties for one or both people having issues. Let them see that the grass is not always greener. Let them see the mountain of PM emails following up about off topic things. Soon they will be complaining to each other about the PM and not worrying about their relationship issues. And if you want to go evil genius, drop some subtle hints that this person's name is being thrown around to advance so they have more of a role in the other two's projects.