6th level warrior would suffice.
Alternatively, if you consider yourself, say, a Com1/Exp3, with a BAB below +6, it's still possible for you to punch with both hands when they make the movie.
PHB p139: "
An attack roll represents your attempts to strike your opponent. It does not represent a single swing of the sword, for example. Rather, it indicates whether, over several attempts in the round, you managed to connect solidly."
So by punching with both hands, you are making several attempts with your weapon (unarmed strike) to connect - represented mechanically by a single attack roll.
(Do be warned that in so demosntrating, you'd provoke an AoO - I've got Improved Unarmed Strike

)
-Hyp.