Lens on the left (his right) is the 600 and the one he's shooting with is the 400 DO. He did have a second body with him...a NIkon D800.
What a nice combination. I am saving for a 600mm to go with my 300mm. I am sure the 400 DO is a really nice lens too. I would want 2 bodies though just to have the option of putting a camera body on each lens.