The pH is approximately
Let's assume these salts totally dissolve in solution.
Starting with the first salt,
and the second,
We may assume the ions will be in equal moles.
Water isn't ionizing, so we may ignore auto-ionization of water. We may organize the equilibrium expression favorably, like so,
From here, we may add these molarities, since there is
And finally, we may obtain the pH!
We are dealing with the 2nd ionisation of phosphoric(V) acid:
These are equilibrium concentrations.
It looks like we have a buffer recipe. Because
Since the total volume is common we can put these amounts straight into the equation for