means that a girls vagina and anus are penetrated at the same time. It does not need to be 2 stiff cocks, no it can also be with 2 dildo’s or one penis and one sex toy.
So DP is also possible between a horny couple having sex. Triple penetration is also possible. But this often involves a second or even third man. It can be very hot to see a girl being screwed in every hole she has.