I'll try to explain it now but I'll post some pictures in a few days of how I do it now. Basically, I buy zipper by the foot now so there's no pull on it when I start. I actually just leave it "on the roll" until I'm done with the first line of stitching and then cut it to length. Then proceed with the second and third lines of stitching. When it comes to the fourth line of stitching on the zipper, I just pull the zipper apart and I don't have to worry about the inside out mess. Once that is complete, put on your pull or pulls as you'd like. I've found that two pulls on smaller pullouts is just kind of a waste but nice on bigger ones.
I hope that makes sense. I've actually got some pullouts ready to sew that I'll take some photos of the process and post.
I'm guessing that's what you are asking but I'm not 100% sure what you mean by adding the pull in the middle.