This is something my girlfriend Rachel and I do that gets us both of off fast. It's also usually almost at the same time. First off, Rachel is 17, 5'1' tall, with 38c breasts and she's really hot.
Anyway, we lie down facing each other and usually play a little first until we're both really turned on. Then she takes my cock and uses the head to masturbate herself by rubbing it on her clit. This way she is masturbating herself, but the act of rubbing herself with my cock jerks me off. The closer she gets to cumming, the faster she goes, which gets me closer to cumming.
This usually ends up with us cumming at about the same time. It's a really fun thing to do and all you couples out there should give it a shot at least once.