“Red Square” was woven on 10 shafts–2 for the foundation warp and 8 for the red warp.
The sewing thread warp I’m weaving on now has a supplementary warp, too. In this case these warps aren’t threaded in heddles, but between them. See more in previous posts.
I weight my supplementary warps with washers that are hanging from shower curtain hooks.
The knot to use (because it is easy to undo and redo as needed) is described in 3 of my books because I think it is so useful. “Weaving for Beginners”, Book #2: “Warping Your Loom & Tying On New Warps”, and Book #3: “Weaving and Drafting Your Own Cloth”. See the chapter, Two or More Warps in Book #2 for details.