Producing the Sum and Difference of two signals with two Op-Amps
You can drive two amplifiers in parallel as long as you don't load the inputs too much (choose your resistor value in consideration of the drive strength of your signal sources).

You need a different summing amplifier circuit in order to get a non-inverting sum, however.  Try a simple voltage divider connected between the two inputs, the result is the average of the input signals \$\frac{V_1 + V_2}{2}\$.  Connect this to the positive terminal of an non-inverting amplifier with a gain of 2.







simulate this circuit &ndash; Schematic created using CircuitLab

The differencing circuit you have now looks ok.