How to handle security deposit with two tenants on the lease

[Aaron K.]

I'm about to collect the security deposit and sign the lease with my very first tenants, a young unmarried couple. The girl makes enough money to rent the property on her own, so I'm tempted to leave the guy off the lease in case they split up. But what if they both want to be on the lease, and I decide to allow that? How is the security deposit handled in this situation, particularly when returning the deposit?