Ooooh!
I think you are talking about two hand slap'n.
Like Grueber said, it's a combination of right and left hand, assuming your right is your slapping (thumb) hand.
Slap the string with your thumb, then come down on the strings with your left hand and make another percussive slap sound by slapping the strings with all four (or three, whatever works best) fingers. Don't fret any notes, just slap the string down to make a percussive sound. Set up your metronome to a slow tempo and practice in eight notes. increase the tempo slightly once you get the sound happening, then try mixing up different rhythms.
Here is a video of a master of slap, Stanley Clarke using that technique. It's quite a long video, about 15 min. Right at 10:53 near the end of his solo he starts laying down a rhythm two hand slapping.
I couldn't find much by google'n but there must be something out there. A great video to get would be Alex Sklarevski's Slap bass Program. He covers that and so much more.
dat it?