Ok so from what I can see from light research Landorus is the responsible one, the Rayquaza to their Groudon and Kyogre if you will. There is multiple ways to see this idea but the one I think is useful for what you want is their abilities, Tornadus and Thundurus both have prankster in their incarnate forms along with the hidden ability defiant.
So how about you make one a prankster and the other a tattletail (defiant), this causes them to fight which makes Landorus have to get involved. I can't say which one should be which but that is up to which one you think can do the best prank.