Is it possible to measure multiple photodiodes on one pin?
You could use an array of analog switches driven with something like a 4017 to sequentially - and mutually exclusively - connect the diodes to the Arduino's analog input. Use a single pin to generate the 4017 clocks, keep track of them, and there'll be a direct correlation between the clock number and its corresponding diode. With a little extra glue logic you could use the 10th clock, say, to hard reset the 4017 after each run and then you'd never get out of sync. 

Here's the basic idea without the glue logic:


You could also configure a shift register as a ring counter and extend the number of diodes sensed.