How to combine PC's power and HDD LEDs?
Seems like it would be simpler to use only one optocoupler, and shunt it with a resistor for the (dim) power-on states. 







simulate this circuit &ndash; Schematic created using CircuitLab

I also think you need more than a 2:1 difference in current for it to be obvious (given the eye's logarithmic response), more like 10:1 for a modern bright LED. You may have to experiment a bit. 

As one data point, car tail lights have about a 10:1 ~ 20:1 brightness difference between running and brake lights.