You've got two guys on a crew who are fighting. John doesn't want to work with Mark. Mark doesn't want to listen to John. Their behavior is running off people. Unfortunately, they are also your most skilled workers and you'll have trouble replacing them.

What do you do?

Well, if I'm in your shoes and I can't split them up, I'd do everything I could to get them to see the BIG picture. Which is:
We are all in this together!

Sit these two clowns down and tell them:
"You two are making things stressful for everybody who works with you, for everybody in the office, and for me personally. Sometimes even our client notice.

"Guys, you need to work together. The other members of your crew need you to work together. We all need you to work together.

"You see, we're all in this together. We all sink or swim together.

"When you're acting like stubborn little children, it holds all of us back. It hurts all of our pocketbooks. It forces me to spend time keeping your crew functioning.