mmm. double thumping has noting to do with 2 3 or 4 finger technique.
Double thumping is only applied in what people call slap.
it refers only to the thumb.
double thumping is hitting the string back and forth with your thumb as you would do with a pick. So actually your hitting both side of the string so there's not really a wrong side of the string to double thump because you use both.
