Usually practiced in homosexual
couples, but it is also done between some straight couples as foreplay.
When you lay down with someone, get in a cuddle type position and mastubate with them.
Some people make out with the person while they do it.
As I laid there, jerking my cock, at the sight of her masturbating next to me, she leaned over and started to make out with me.
This mutual masturbation thing is great!