It takes two kits to bleach a buffalo skull.
Also, you don't have to cut off the horns. They are caps that will release during the boiling process. (Don't worry, there is no bleaching agent in the boil so the horns won't be discolored.)
Once you are done bleaching the skull you can reattach them with some plaster or sheetrock mud.
Just spread the compound on the bone and slide the cap back into place. Allow to dry.
Some people will polish the horn caps, but usually I just clean up the bottom edge.