Depends on how your jacket was originally constructed. On most, you will stitch outer and inner layers together only through the curved area. Once the fabric(s) are being hemmed in a straight line, typically at the side or front side seam, you'll want to keep them separate. This allows the lining to move independently of the outer layer. Here's a link to one post in a thread where I showed an entire conversion -- it shows the detail I'm trying to convey.