Sorry hurt my back and have not been building or pay much attention to the forums my plans obviously have slightly diffrent numbering since i am building a Y tail and not the Standard tail but from a wing perspective if i recall correctly you drill the other block when you mate both spars, check the drawings down the tree, there is a point where you mate the spars prior to attaching the lock in blocks that look kind of like missoury but less curvy, and you use the 3 alignment holes to do that well part of that process is also drilling out the second block with both spars bolted together temporarily,
Granted i am answering this from memory but check down the drawing tree you should see a page that goes over that.
Found it
https://eaabuilderslog.org/?blprojentry&proj=7Wc0KY8D6&e=7y2oXlsjv&listcat=wings&sid= there are some pics of the mating process and the follow on drilling of of the second attachement block using the drill guide that is 1/2 inch in OD which is why you need to drill out one in a drill press first so you can fit the drill guide.