The earliest firearms didn’t use granulated powder, but finely ground. Previously, I have posted a link about slow burning powder, where a guy (the same one as in these clips?) argues that a gunner had the time to ignite his powder, take aim and wait for the actual shot.
These movie clips show that principle, which indeed makes it possible – if not probable – that handgunners acted alone, although other evidence suggests they also could have acted in pairs.