[just another ref]

Originally Posted by ilyazhito

Officials can come together if both raise a fist, but once the block and charge are both signalled, in NCAAM or NFHS rules, the calls cannot be withdrawn. This, therefore, is a situation where an ounce of prevention (using proper signals to stop the clock and scanning the floor before making a preliminary signal) is worth a pound of cure (enforcing both fouls).

There is no mention of signals, whether it be the fist, the specific preliminaries, or anything else, in the NFHS rule or case on double fouls, but apparently that's the way most officials do it.