It's really like duplicate-stitching a row, just between rows not over an existing row. That way of doing it is very odd and personally I would use a provisional caston and then could graft the two rows while live (you need the same stitch number remember). If you have already knit it I would recommend undoing the caston row before, and then graft (live stitches again). Either that, or graft some practice stitches until you are a grafting queen (by which I mean really understand what you are doing and why and where it goes on the final product) before you try with caston row.
Or pick up as for a destination row, push the caston edge out of the way, and treat them as live stitches.