500 ohm relay coil seems very high but, if that's correct, they will only draw 10mA.
In that case, you don't need to double up the uln2803 drivers. just one should be ok. each one can drive more than 10X that current.
Your opto's look wired wrong. there should be a limiting resistor in series with the LED. the transistor should be wired shown in the attachment a wire from the collector going to the uln2803. ignore the R1 and the +12V connection for the lED, that was drawn for a different application
Also, I don't understand why you have extra LEDs and what they are attached to.
However, you might just get away with using the opto's transistor to directly drive (er, pull) the relay coil since they are pulling 10 mA
If you haven't breadboarded this circuit, I would do that before getting a pcb made.