Without going into merits of the two-piece barrel, and there are many, the term "two-piece barrel" is a misnomer.
The barrel is still in one piece, a rifled tube.
The second "piece" in the current arrangement is the shroud, which is affixed to the frame by fitting over a "key" so that the sights are held straight up and down. The shroud is held in place against the frame when the tube (the rifled part through which the bullet passes) is screwed into the frame at the cylinder end, and the other end of the tube screws down tight to a specific torque setting against the muzzle end of the shroud, holding the shroud tight to the frame.