Switching regions for a skin placeholder within the same skin would be duplicating how slots work. Slots slots slots, all the way down.
Each placeholder has its own attachment. If you have two skins and both have a "jeans" attachment, you actually have 2 independent "jeans" attachments.
You could use an animation that only has keys on frame zero solely to configure which attachments are visible. Nothing wrong with that, though you don't get the benefits skin placeholders provide for reusing animations that have attachment changes (see docs).
You could try duplicating the skin, which will duplicate all its attachments, then deleting the attachments you don't want. Or you could create two jeans attachments as you have been, but go to the first, ctrl+c, go to the second, ctrl+v to paste the attachment's transform.
It seems you are using skins solely to configure what attachments your skeleton has. This is usually ok, but if skins aren't a good fit, it's not a super hard problem and can be solved other ways. Eg, it could be done at runtime, maybe using a text/JSON/CSV file that describes what attachments should be used per outfit/item/etc.
