With Aowyn and my in laws lab I let them be and do their own thing until i feel it is getting out of control. Usually its just a matter of clapping my hands and in a loud but forceful voice i say "ENOUGH" and then both stop and look at me immediately. Its just a matter of finding something that will snap them out of their current focus.

Other than that i stay out of it. Sometimes its good for the older one to put the younger one in its place. I have never seen it escalate more than one saying they had enough.