I think it should be water, proportion of H2O : BaSO4 = 1 : 1.
Ion of HSO4, consist of H(+) and SO4(2-).
In water, the ion SO4(2-) will be separated, make a precipitating reaction in ions: Ba(2+) + SO4(2-) -> BaSO4 (solid).
Means if met water, the reaction would likely be: Ba(HSO4)2 + H2O -> BaSO4(s) + H2SO4 + 2H2O.
Also explains why Ba(HSO4)2 don't exist in water.
If this explanation correct or not, just reply, I'll make some change in my things, because people always have their own mistake. :p