How can I sum two PWM signals of different frequency with a Op Amp?
You could try a non inverting summing amplifier.



https://masteringelectronicsdesign.com/the-transfer-function-of-the-summing-amplifier-with-n-input-signals/

You put your two input values on the (+) input. The link also provides the output equation as



If you make all the resistor values the same it should come out as Vout = V1+V2