(PROTIP: Not actual rape.)
An act involving three people; one victim and two instigators (let's call them the rapists). The rapists position themselves opposite eachother, with the victim directly between them (the victim can be facing any particular angle, but it is most effective when victim is directly facing one of the rapists). The rapists close in to "sandwich" the victim between them, effectively raping them.
See also the following variants:
and many others.