That would work. You would have a hard, permanent, never to be crossed fence between regional and mainline equipment, pay and workrules. But, everyone would be on the same seniority list.

Newhires from the military or civilians with substantial experience in international ops and/or large aircraft (over 100,000 lbs) would go directly to the mainline and those from other regionals or from puppy mill aviation universities or flight schools would go to the regional side. That way career expectation would be maintained.