This intrigues me, using a seam ripper never seemed to happen in a certain way. But now that I think about it, my preference is to pick carefully between the two pieces. By carefully, I mean don't pull the thread or the fabric but gently cut the thread three or four stitches away from the opening then loosen to the cut and continue on. I find that this way I end up with one thread not cut making it easy to pull out one long thread. It also makes the thread obviously different from the fabric.