This is more of a garment construction issue vs. a quilting issue. Garments, even ponchos, are meant to drape on the body. You have to be careful about the interfacing you use. If you use a non woven, even a featherweight, you will loose the draping quality of your knits. There is fusible tricot knit that would maintain the knit qualities, but it's not double sided. Even if you could find a double knit, the interfacing would pull away from the fuzzy side of the sweatshirt fabric due to wear. Personally, I would just quilt the 2 pieces together after pinning the heck out of them. It's a poncho, not a fitted garment, so if one layer shifts a bit during quilting, just recut the all edges to match.