Indeed - And AA / QF already have codeshares in place
For example, AA has flights 7341, 7296, 7316, 7309, 7382 & 7412 from Sydney to Melbourne today .also 7351 to CBR, 7343 & 7337 to ADL, 7354 & 7332 , plus others too
None of this requires collusion on revenue sharing on the routes to make them effectively operate as a single airline in the region