Is there a way to make a cast-on that has two knittable ends? If not, is there a way to join 2 pieces of knitting by their cast-on edges seamlessly?
There is double knitting, with 2 right sides that uses a double cast-on. Like a sandwich with the “wrong” side inside.
There is also provisional cast-on. Where you knit in one direction (say up). Then come back & remove the base stitches, and work in the other direction (say down.) I think you are asking about provisional cast on.
Let me look for a link to show you how. I think there is one on the banner of this site.
This might help. Go down the provisional cast-on link.