What type of cardigans are you making? I'm understanding you to say that you are making the bands separately and sewing them on. Is that correct? What is the shape of the neck opening? Is there a collar of any kind beyond the band itself? I'm just trying to get the whole picture. I was also wondering if there would be another way to do the bands and neck than the way you are doing it. Maybe in your situation there isn't.
If you are seaming two bands that are made separately and sewn on, up the fronts and joining at the back neck edge, the best thing I know to join them is to use grafting, or kitchener stitch. That can be invisible, but it is also tricky to do in ribbing, or any other stitch that changes from knit to purl. If you are doing garter stitch it wouldn't be that hard.