During a threesome, when you receive a blowjob and rimjob (getting your rectum licked) at the same time.
The term "double shift" is derived from work shifts; as you receive two jobs, you are working a double shift.
Can also be used in past tense and as a verb; I got double shifted last night. Or I love double shifting you.
Mike: Hey V, what happened with you and those two girls yesterday, Genoush and Vartuhi?
V: Bro, everything was going good, I was making tolma with them in the kitchen then they randomly took off my pants. Genoush sucked my dick and Vartuhi ate my asshole. It was weird man.
Mike: Oh, your so lucky, you just got a double shifted.