Double thump can be concepualized differently and a simple search of "double thump bass" or "double thump bass lesson" yeilds a lot of video responses on youtube or google.
One way to concieve of double thump is think of the thumb as a pick. Placed horziontally to the strings, the thumb can strike down through a string (say the E string) and rest on the next string (the A string) before striking the E string again (this time with the back of the thumb) on the bounce back up.
Because of the dynamics which can be achieved and the natural popping position that the other fingers are left in, I use this almost exlusively for my slapping. The dynamics allow for tones other than slapping though, when mastered, the double thump can emulate a pick, tremelo, fast fingerstyle...
