the 5-5 comes from the Revised Most Powerful Battleships National Advantage for Japan.
The firing twice in the first round comes from a Revised Larry Harris Tournament Rule revision. The NA was titled Naval Night Fighting Skills, yet only applied to the battleships. Personally, I don’t think it was named well. I simply fuse the two.
Also, I think most house rules I’ve seen give SBBs 3 hits. I considered keeping it at 2 hits, but after I gave my supper carriers 2 hits, SBBs needed 3.