multimeter input impedance calculation discrepancies
Consider this circuit: 







simulate this circuit &ndash; Schematic created using CircuitLab

Vx will be 4.9975V or only about 0.1% different from if R3 was infinity. You can't expect to get good answers with such a small "signal". If R1/R2 are not perfectly matched that mismatch will dominate. 

You can improve the results by swapping R1/R2 and averaging the voltage readings before calculating, but your meter resolution will also cause issues. Don't be surprised if you get a negative answer for one of the meter resistances if you calculate without averaging. 

You might want to plot the results for different resistor pairs, swapping them and averaging them and seeing how it converges to one answer as the resistance increases. 

Or simply do as @Tony suggests and measure with and without a resistor in series with the meter. R2 doesn't really help you in this case and introduces an additional error source from the nonlinearity in the meter (in fact you might use this circuit to measure the nonlinearity).