I sew a lot of twin needle hems on knits. I loosen the upper tension slightly & use a Schmetz stretch double needle. A stretch (not ballpoint...stretch) needle on knits will stop the skipped stitches. I usually use a size 75 / 4 and never have a problem o nearly any weight knit. They're not hard to find....Hancock's has them or you can buy online.
Paper under the fabric is a great tip for the rippling & sometimes a walking foot will help. So will lightening the presser foot pressure.
Nancy Zieman recommends NOT separating the threads til you get to the thread guides right above the needle....so I usually run both threads thru the same tension discs & separate them at the guides above the needles.
I do my hemming on a vintage Kenmore.