I just learned about this seam Bickford Seam
that makes a seam that is invisible on each side. Then there is usual mattress stitch. I don't know if either of those work in your case. It may be more like sewing a sleeve to the body of a garment. That would be stitches to rows and here is a link to some help on that. StudioKnits
That is part of an online knitting book that is free and there is a lot more about seaming there.
It depends on the direction you knit the edging in which ones will work best where.