I've not tried the double knot shown. I did try the braided join
and used it couple of times but went back to the knit 2 or 3 stitches with both yarn ends and leave tails to weave in method. Even with cotton or acrylic I've never had it come loose and some of my acrylic and cotton creations get some really rough treatment, such as dish rags. If you're talking about slippery more like Simply Soft, I think the double knot might be the way to try. Let us know what you think of it if you try it, please.