Sanosuke with his futae no kiwami used to punch holes in the walls, break doors, etc. But Makoto Shishio just with his head not only could take it but he broke Sanosuke's hand. Shishio said something to Sanosuke that before he attacked he should have watched and learnt what to do, or something like that. How did Shishio to break futae no kiwami attack?
Well as it was stated in the anime and manga, Shishio has the unique ability to neutralize any and all techniques he's seen before. So having witnessed Monk Anji multiple times over the course of his servitude use the many variations of the Futae No Kiwami, it would stand to reason he knew how to counter the technique.