Two phototransistors in parallel don't have the impedance I would expect
Since phototransistors are not linear devices, and you are not giving these any power to begin with, they are not acting linearly.  Sometimes the approximation is valid, for example when biased in a near-linear region, but just floating like that they are not.

I suspect that if you biased both phototransistors (using like a 1-10k resistor and a 5V source) that their impedance would respond more closely to how you are describing.