So we can safely say it is not fur loss due to old age.
One of the main reasons for hair loss around the heads of mice is, barbering, where a dominant mouse chews off the hair and whiskers of the other. So if there is a mouse with its facial hair still intact, this maybe what is happening? You have done right in separating them. Keep them separated, and see if the fur starts to grow again, this will only take a week or so.
The other thing it could be is Mites, these usually start around the head and muzzle, then eventually over the whole body, but persistent itching normally accompanies this, and this would affect both, keep your eye on the other one. But it sounds to me this is barbering
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