Hi Power Two-Piece Barrels
I'm aware of no one-piece barrels for the Hi Power that came from FN after the early '60's. So far as I know, they've all been two-piece, at least since the time in which the locking lug dimensions were changed. I do have a couple of early Bar-Sto match barrels that are one-piece, but later ones are two-piece. I've noticed no increase in accuracy of the one-piece barrels over the conventional two-piece barrels, nor have I noticed them coming apart or being less durable than the one-piece barrels.
In this picture, you can see the seam on the FN factory barrel. I have no exact reason for the use of two-piece barrels, but suspect strongly that it was for ease of manufacturing.
Some years ago, a gun magazine did a test on various 1911 barrels that ranged from GI surplus to the match barrels from some really good makers. They fired the barrels via a device that locked the barrel firmly in place. To cut to the conclusion, there was very, very little difference between any of the barrels in terms of their purely inherent accuracy. I believe this included a two-piece Springfield Armory barrel as well as some other factory barrels.
There are reports of Browning Hi Power barrels failing after a great number of shots, but I don't recall any of them separating. There very well may be some such incidents, but I'm betting that they're statistically rare. With as many Hi Powers in use around the world, if it was a real problem, it would have been addressed by now.
I don't think it matters.