Add two PWM to DC inputs
Transistor is correct in his interpretation. I have a situation where my two filtered outputs go from 0 to 5V in N steps. I was trying to figure out a way that I could combine the outputs to create a single DC out with 2N steps (presumably adding them to a range of 0 to 10V).








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A non-inverting summing amplifier.

How it works:


The voltage on OA1+ input will be \$ \frac {PWM1 + PWM2}{2} \$. 
The gain of the non-inverting amplifier is \$ 1 + \frac {R5}{R3} = 2 \$.
Output will be \$ PWM1 + PWM2 \$.