No there are two of these circuits, and when both outputs are connected in a passive mixer I get two modulating square waves.

Petkan:
It appears you are collector ORing the two signals. As they are synchronized albeit with different duty cycle, the result should be periodic corresponding to the OR of the two (Low when one or the other is low)

 

...the result should be periodic corresponding to the OR of the two (Low when one or the other is low)

Wouldn't that be AND?

OR would be low only when both are low.