I have a flat-felled generic snap on foot for my older Janome.  I prefer to do it by folding and pressing the seam by hand.  At first I was thrilled to get this foot as I made a lot of cute boxers for my grandson when he was young.  It might be good to note that it can no longer be used with my machine as I have up update and I don't believe there is or likely will be an ankle that fits the new machine and attaches the slimmer feet. I did not use the foot for jeans either.  I found a much better result by sewing the seam, clipping, then pressing, then top stitching.  I would not use a double needle on denim.