Surely there's already a ticket for this, but I couldn't find it.

Right now if a release needs to be split into multiple mediums, you get two edits: one that is not an auto-edit, to remove tracks from the exiting medium, and one that IS an auto-edit that adds the new medium. You also have to relink all the recordings that were removed from the existing medium.

So a split medium edit would do two things: it would retain the recordings when you're making the edit, so you don't have to mess with relinking them, and it would enter the edit in such a way that you don't get tracks duplicated on two mediums until the original medium edit is applied. (Edit: well, and presumably it would make sure that if the change is rejected for some reason, you don't accidentally end up with a bigger mess because one edit passes and another doesn't)