Is there any diprotoic acid where both pka's are less than 0? The question states it all. 

I'm working out special cases for a different problem and such an acid would create another special case. 

Sulfuric Acid is the strongest that I remember. pKa's −3, 1.99
Ethanedisulfonic acid does indeed have $pK_{a1}=-2.06, pK_{a2}=-1.46$.
Using polycyano substutution can also work.  This paper reports the table below, note that the tetracyano hydroquinone &quot;makes it&quot; (in the excited state produced by irradiation).