double power supply for optocoupler (is it necessary?)
You're not providing two power supplies to be able to use the optocouplers, you're using optocouplers to be able to communicate between isolated circuits. Optocouplers are bulky and expensive, so if you could avoid using them, you would almost always prefer not to.

The optocouplers are used here to provide safety isolation between a high-voltage circuit and another circuit, since the "output side" could all potentially be connected to 220 V in a fault situation. 

If you were to combine the +5D and +5A nets (and the two ground nets), then there would no longer be safety isolation between the two sides, and your users could be electrocuted due to a fault in your circuit.