How are you going to power the Arduino before you turn the power supply on? Via USB from the RPi? I think you'll have issues as the RPi doesn't have much current capabilities via it's USB port, especially if you're also using wifi or have a camera attached to the RPi.
The route I took was to use a cheap 4 port powered USB hub, cliped off off the wall wart and instead spliced the power connector to +5vsb. One usb cable goes from the hub to the RPI micro USB power connector. This obviously provides the power for the RPi. No data goes across this connection. Then from the RPi USB Type A connector, another connection back to the hub. From there, I have a connection that goes to the Arduino. There's plenty of power left that I can also have my USB camera and EDIMAX wifi adapter plugged in at the hub.
Before the powered hub, I had intermitten issues when setting things up if I tried to use both the camera and wifi at the same time. Plus I had to have a hub anyways since I needed 3 USB ports and I wanted to keep everything with just basic USB connections.
The route I took was to use a cheap 4 port powered USB hub, cliped off off the wall wart and instead spliced the power connector to +5vsb. One usb cable goes from the hub to the RPI micro USB power connector. This obviously provides the power for the RPi. No data goes across this connection. Then from the RPi USB Type A connector, another connection back to the hub. From there, I have a connection that goes to the Arduino. There's plenty of power left that I can also have my USB camera and EDIMAX wifi adapter plugged in at the hub.
Before the powered hub, I had intermitten issues when setting things up if I tried to use both the camera and wifi at the same time. Plus I had to have a hub anyways since I needed 3 USB ports and I wanted to keep everything with just basic USB connections.