Roscoe, so far as I know, he just held the trigger back and played slide trombone. It obviously takes a bunch of practice--but doesn't everything? IIRC, he was timed at 2.7 seconds for seven shots--all hits. (If I'm wrong, it ws no more than 3.4 or 3.7. G&A had an article several years ago.)
Satterwhite showed up at the SOF Convention one year, and entered the "Rolling Thunder" event. His toy was a pair of Remington 1100s mounted together with a single trigger and buttstock.
Art