Complex sentence: Having the two guns firing simultaneously would obviously not have been a satisfactory arrangement; the guns needed to both fire "at the same point on the propeller disc", which means that one would fire a tiny fraction of a second later than the other.
Split into simpler sentences: Having the two guns firing simultaneously would obviously not have been a satisfactory arrangement. The guns needed to both fire "at the same point on the propeller disc", which means that one would fire a tiny fraction of a second later than the other.