Why we use two photodiodes as optoisolator or optocoupler?
I think you might be referring to an analogue signal opto isolation technique like this: -



The op-amp on the left is driving an analogue signal across the optical barrier and, to make sure the photodiode on the right is getting the correct signal level, it uses a local reference photodiode and senses the light it emits. In that way, if temperature (for instance) affects the LED light output, it can be compensated for.

They are pretty good (better than 1% linearity in many cases) but not as good as digitizing the signal and sending logic streams at high speed.