Next month we will start 3 parallel projects and we hired 10 new employees to help us.
For each project, I already have a Leader but they started to fight to get the best new employees for their teams.
How can I help them to divide the resources?
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If you are the decision maker as to which team each new employee goes, then I would suggest to remove the team lead from the equation in this situation and here's why: In this situation, since each was hired generically and then are to be assigned, I would remove the team lead in choosing, so as to avoid the fight you are experiencing and because their assessment of who is truly "better" is unreliable and based on bias. Unless you used some type of "job testing / job simulation" hiring vehicle, you and your team leads have no real reliable and valid way of knowing who's going to perform and who won't.
You'll end up using years of experience or something on the resume that sounds interesting but both are extremely unreliable predictors.
So, using your best judgment, biased notwithstanding, divide the team as you see fit and tell the leads to cope with it. Even better, use a coin. I'm really not kidding. Our job predictors we generally use, unless you have a proven job testing method, are extremely weak and not much better than random choices, if better at all. Your goal here is to stop the fighting, which is a waste of time, so pick up a coin or just you split them up.
These are all new employees. Unless your team leads are good at sharing with each other, much better to leave them out the equation.
The situation in which you have to assign several people you don't know well (or at all, as in this case) to different projects comes up often. You can avoid a lot of friction by making a best guess (as already suggested) yourself, and then adjusting if needed.
What you don't want is for the Team Leads to get emotionally invested in specific choices (as in, "that's the resume that makes most sense for my project") before they have a chance to work with the assigned team--that way leads to unhappiness regardless of how things work out.