If you have two Ka's, wouldn't it be just like if you had a diprotic acid? Couldn't you calculate the overall pH based upon the two different Ka's you have?
No idea what two Ka's you are talking about
As I see it Ka is a function of solution composition, with highest value for 100% H2
O and lowest for 0% H2
O. Perhaps dependence is linear - I doubt, but then it may be so close to linear that it doesn't matter. Knowing Ka as a function of - say - water molar fraction, one may try to do the classic calculations of pH for weak acid. As we don't know the dependence between Ka and solution composition whole idea is academic.
But Ka is only part of the problem. What happens with activities in the solution containing other than water, less polar solvent? Activities are partially dependent on electrostatic interactions between ions, these will depend on dielectric constant which changes with the solution composition. No idea what will be the outcome.