Is there any diprotoic acid where both pka's are less than 0?
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).