I spent a couple of hours examining WWII barrels by Hi-Standard (who supplied Remington-Rand, Ithaca, and US&S) and Colt, who were the major maufacturers in WWII. I do not have barrels by Flannery, Singer, or Springfield, but perhaps those who do could look for evidence of two-piece construction.
I can't find any evidence of two-piece construction on HS or Colt barrels. If there are lines, they are well hidden and the tooling marks do not bear out two-piece construction.
If anyone has further info as to the maker and markings on those WWII two-piece barrels, I would appreciate your passing it on.
As to making barrels that way, it is done now by several makers, and post-war BHP barrels are two-piece. There is nothing wrong with the technique and the maker can start with seamless tubing, a much less costly system than forging or turning from bar stock.
I do note a story on Kimber which has pictures of barrel forgings, so they don't use two-piece construction.
Jim