Instead of having to fly around and watch them slug it out, make them break up the battle and face you one at a time if you choose to interrupt.
Ideally there would be 3 way battles. If that's too difficult, then something a bit weird like making them instantly break it up and fight you either one at a time or together is preferable to sometimes having to wait weeks for the battle to finish.
Having them both attack you together is, in my opinion, more natural and conducive to interesting gameplay than just having to wait their battle out.