"Why is it firering in both directions?"
I suspect it is a demonstration. Note the gun barrels are almost horizontal. That is point blank. If you have to fire simultaneously on both beams at real targets, you are in deep doo-doo.
Usually, capitol ships engage their targets at long range, to reach out that far, the barrels are elevated. (That is one of the reasons the Hood was lost. Her deck armor was designed before the engagement ranges were great enough that the shot was dropping through your decks rather than coming through the side. The British found that out much to their chagrin at Jutland, but the Hood's design was already set.)
Les