If you’re using Photoshop 2020, you might have noticed that Warp (Edit>Free Transform>Warp) now has options to split the mesh vertically or horizontally. If you Right-click inside the Warp bounding box, you’ll see several options for splitting the Warp. These same options are also in the Options Bar at the top. But you don’t need any of that; simply hold down the Option (PC: Alt) key and you’ll see those split grid lines appear at the point of your cursor. If you move your cursor close to an existing grid line, the option will change from crosswise to either horizontal or vertical, depending on the direction of the line to which you’re closest. Just click to add the grid lines.
This tip previously published in Colin Smith’s Photoshop Tips column, in the January, 2020 issue of Photoshop User magazine.