[Reaper]

I was thinking you could use Rare earth magnets (those super strong ones) to attach the cape to the tunic. You could create a small pocket for one set of magnets on the inside of the tunic. it could be just big enough to place your magnet in and then seal the magnets in, or you can make it a small pocket with one side open so you can remove the magnets (I'f face the openings to the sides, left and right, but that's just me lol)

On the cape itself, you can seal the opposite magnets into thelining of the cape. Place one magnet no top of the other, and you should have a solid hold with no visible attachments.

This way you can use the same tunic for both costumes.