What happens to the security deposit when you rent to roommates?

[Ellie Narie]

If you rent out an apartment to two unrelated people (roommates/friends), but they're jointly liable on the rental agreement, what happens to the security deposit if one roommates decides to move out? 

I assume the remaining roommate gets another roommate... what if the remaining roommate moves out, and then the only tenant left is the new roommate, who wasn't even on the original lease... who would I give the security deposit to? 

What would be your recommendation to renting out an apartment to two unrelated people?