I've only recently been applying myself to double thumbing as a technique in itself.
I've not yet got it down to my satisfaction but it helps me to get a better feel with some of the lines I practice. I'd imagine it to be as fast as playing with a plectrum when done by qualified adults under the guidance of the local safety elf just as soon as he receives the risk assessments signed off in triplicate. I can see why MK uses insulating tape. It's a technique that requires toughness and stamina when used consistently
It's never going to have the clack attack of a plec but it's good to have the knack never the less. *Try saying that out loud with a gob full of granola.
*Mild choking hazard and strong expletives to be expected.