You gotta get your mind right youngblood!!!
1st of all do you want to Thump or Slap? If it is the latter then get out your string bass & we can continue from there... However, if you want to double THUMP then 1st realize it is 2 different techniques...
If you watched the 1st Victor Wooten tape (yeah, it was that long ago!! lol) He explained that the purpose of the double thumb was to play more notes with less movement & energy expended...
The trick is to deliver a glancing blow into the string & downward, then after your THUMB passes the string from your initial articulation it comes back up and plucks the string again. You thumb is basically acting the way a guitar pick would (I know that turns some of you off, but thats how it is.. ;-) ).
This may come easy to you depending on how you are already attacking the string. If you were more or less Thumping in the Fonzie position it may take some adjustment of angle from the 90 degrees to the 45 degree or less.
And as the other folk said... It takes practice. Dont despair!! And whatever you do, use these powers for good & not evil!!!