Multi-ply web forming stock jets directed through a slice from a single headbox into a forming zone between converging forming surfaces in a papermaking machine are maintained separated for a sufficient interval after leaving the slice opening to permit drainage and partial ply formation of the stock delivered by one of the stock jets to one of the forming surfaces before the stock in a second of the stock jets joins in fiber felting laminar relation with the contiguous surface of the partially formed ply. Flexible divider sheet members divide the slice chamber of a headbox into separate flow passages aligned respectively with different ones of headbox subchambers to receive streams of the stock from the respective subchambers and maintain the streams separated, and the flexible members extend a substantial distance out of the slice opening in the direction of flow of the stock jets to maintain stock jets separated for a sufficient interval after leaving the slice opening.