Calculate ion concentrations in mixed buffers
You can ignore K+, Na+ and Cl-. You have to deal with H2O, H3O+, OH-, Ap3-, HAp2-, H2Ap-, H3Ap, Ac3-, HAc2-, H2Ac-, H3Ac. Here H3Ap = H3PO4 and H3Ac = citric acid in neutral state. You have 11 unknowns.

(+1)    [$\Delta$H+] = 3*[citric3-]+2*[Hcitric2-]+1*[H2citric-]+[(PO4)3-] - [HPO4-]-2*[H3PO4]
(+1) [H(from_water)+ + $\Delta$H+][OH(from_water)-]=10^-14 (dissociation of H2O)
(+1) [H(from_water)+]  = [OH(from_water)-]
(+3) Three dissociation constants of H3PO4 are known
(+3) Three dissociation of citric acid are known
(+1) Concentration of [Ac3- + HAc2- + H2Ac- + H3Ac] is known
(+1) Concentration of [Ap3- + HAp2- + H2Ap- + H3Ap]is known   

You have 11 equations.  

OH(from_water)- corresponds to total OH-
H3O+ corresponds to H(from_water)+ + $\Delta$H+]  

//(+1) Water concentration is assumed 1 in solutions      

Forget about (PO4)3- and H3PO4. They will have negligible impact on overall pH.

You didn't tell the ratios of Na2HPO4 and citric acid and these ratios are critical. Given pKa of citric (3.15, 4.77, 5.19) and phosphoric (2.15, 7.20, 12.35) acids you will most likely deal with either of these regions:
2.15-3.15 H3Citric + H2Phosphoric-
3.15-4.77 H3Citric+H2Citric-
4.77-5.19 H2Citric- - HCitric2-
5.19-7.20 H2PO4- - (HPO4)2-  

If you know which interval you are working with (check ratio of the reagents, aka stoichiometry) then you can pick the relevant equilibrium and set all other concentrations to zero for initial evaluation. 10^-15 M of ions will have zero impact on overall pH.



I regrouped some equations and expanded the one you asked about.

Can you solve it for a general case. Yes, you can. But it is like accounting for air friction, Special and General Theories of Relativity and quantum effects. It is harder, but it does cover more cases. In this case you will be dealing with x^11+a*x^10+...=0 instead of a square equation. You can solve it using Wolframalpha. But at the end of a day you will find that in a given region the impact of majority of members is negligible. Think of it this way. You can approximate the system with a spline (5 polinomials of second degree) or one polinomial of 11th degree.