Re-aligning the two halves of the fipple is the secret behind re-voicing a whistle. Straight out of the box whistles like Feadóg, Generations and Waltons etc. have the edge of their lips pointing too high into the airstream from the windway-exit. This is responsible for the tendency towards squeakiness and the rasping sound character so many whistlers complain about.
Re-voicing could be done,-and is in fact often done by some tweakers by filing off material from the top-side of the lip, and adding some suitable material to the under-side of the lip. Now the lip will be located at a lower point in the airstream. I chose a different method of re-voicing which involves splitting the fipple and then re-aligning, simply because it's a quicker method, and because it ensures the very same result from one whistle to the other.
Bonding the two halves is not done by gluing, but by welding the parts together using a solvent. This produces a strong bond, much stronger than by using any kind of adhesive.