How do I divide damages between departing cotenants?

Stop worrying. If both tenants were on the rental agreement, both are responsible for leaving the place clean and undamaged.

Best of all, it is not up to you to attempt to divine who did what when. That is courtesy of a legal principle known as joint and several liability. It simply means that any one of your two cotenants may be required -- by you or a judge -- to pay for all the damage and even all the rent. How the two cotenants split up the responsibility and the attendant bill is between them.

When the second tenant moves out, conduct your inspection and deduct from the deposit only those sums necessary to cover unpaid rent and damage and cleaning beyond normal wear and tear. You can safely split the balance and send half to each. Or if you cannot locate the one, you can give the entire sum to the other whose whereabouts you know.

This ought to teach your tenants, especially the newcomer, a valuable lesson. When joining an existing tenancy, it behooves the new tenant to ask the landlord to conduct an inspection when he or she moves in, to avoid being charged for preexisting damage. Or, at least have a candid discussion with the pre-existing tenants about damage that the newbie sees at move-in. A written agreement among all tenants, in which the existing tenants agree to cover the damage, will be invaluable evidence for any tenant who ends up paying the landlord and wants to go after ex-cotenants in court.