Determine the pKa of a diprotic acid by titration (if pKa1 is similar to pKa2)

Determining the acidity constants of a diprotic acid (in my case, tartaric acid) by titration with NaOH and pH-meter is not possible if pKa2 - pKa1 < 2.


This is because there are no "pH jumps" at the equivalent points, so you cannot exploit the method that uses the "midpoints":
[![enter image description here](https://i.stack.imgur.com/Zw97T.jpg)](https://i.stack.imgur.com/Zw97T.jpg)
In fact, at pH=pKa1 the concentration of H2A is not equal to the concentration of HA- (the simplification performed in the equilibrium constant equation is not valid), as can be seen from the speciation diagram:
[![enter image description here](https://i.stack.imgur.com/aox1a.png)](https://i.stack.imgur.com/aox1a.png)


This is how the titration curve of 1.5 g of anhydrous tartaric acid VS 1 M NaOH looks like.
[![enter image description here](https://i.stack.imgur.com/atr1A.jpg)](https://i.stack.imgur.com/atr1A.jpg)


How could this be done?


