No, that wouldn't really work. Firstly the Focus Sash only works when the holder has full HP and gets hit with a move that would KO it. If you have two level 1 Pokemon a OHKO would be unlikely (you'd be using Quick Attack).
Secondly, Endeavor wouldn't work either. The point is bring your opponent's HP of 300+ down to yours of about 20 (or 1 HP if they hit you and the Focus Sash activates). If both have 20 HP nothing will happen.