How can a manager handle a hard situation between two employees, who are working closely in a project ?

When this is happening, the personal is overshadowing the professional.  Therefore, a personal approach is the best way to start - forget the project for a minute and just focus on the individuals.  Why have they gone from colleagues who are simply disagreeing about the best way for a project to proceed, to people having an argument or falling out?

If that doesn't work, you may need to intervene to make a decision about the point of contention.  This may cause resentment, but you can deal with that separately once the project is back on track.

If all else fails, remember that you are the boss. Fire one or both of them, either from the project or the company.  You don't need people who can't get along in a team.

Forget that they are employees for a second, how would you handle any situation like this,2 friends,2 family members etc. you have to listen and you have to be diplomatic, you have to speak to them individually and make it clear that the priority is not them but the project they are working on.