The earliest this can be done easily is the second mission, Hijack at the Hairpin. Have your co-op buddy get killed by the first enemy, then kill them back with your own shotgun. Just like any other interaction, walk into your co-op partner to revive them. As a reminder, this must be done over Live, not locally.
On later missions, guards can kill you a lot faster, so it's arguable that this can come with regular play on later stages if you're playing online.
Also, more of an FYI than necessary information, but the Redhead revives twice as fast.