Why wouldnt you share grounds and 5V? That means 12 pins. I just did this calculation for my project and I'm going with a U-HID nano and a minimus AVR encoder..
The nano and AVR setup might work for this project too, but there are a few design limitations:
1. 30mA limit on 5v and ground pins for the U-HID.
2. Just one wheel and one pedal will use 5 pins (5v, ground, data A, data B, and wiper) -- the two LEDs in the optical circuit probably draw about 30 mA. (most LEDs draw ~15-20 mA each so
check how much current your circuit draws)
3. Adding the second wheel and pedal to the nano causes you to either:
- Run out of pins (5*2=10)
- Draw WAY over 30mA by sharing 5v and ground between the two optical circuits
The way to get around this is by using 5v and ground from the AVR to power the LEDs in one (or both) of the optical circuits, depending on how much current they are drawing.
Scott