How to double penetration
It made me
CUM 
"In the Navy had this regular thing with a guy and his wife. She suggested dp, we went for it and she LOVED it!"
How I did it: She was on all fours, and we took turns being on top (in her ass) and being on bottom (in her pussy). She loved it and wanted to do it almost every time we got together. Also led to some other experimentation, such as double vaginal penetration.
Lessons & tips: Just make sure that the guys are okay with sharing a woman, this can complicate the situation if one of the guys can't handle it.
