Supply Arduino + Raspberry Pi3

I dont know if this is the right place to ask about that. I need to turn my project portable, so i need to supply my arduino and RP3 with the same battery. Im thinking about use two 3.3V lithium batteries in parallel with a step up booster (Module XL6009) to 5 volts. Other solution is to use a 7.2v nimh with a step down to 5v. I want to supply RP3 with a battery and, when i plug arduino to RP trough usb supply this to. By the way usb port will be used to comunication to.What you think?
Best Regards,

If your hooking to a USB from laptop you won't be powering the Arduino with a battery unless the USB cable has been modified to remove its power wires and has signal only wires. Otherwise you have two sources of power trying to feed the Arduino.

You have not said what Arduino you are using. Why not get one that runs on 3v and power everything from a pair of AA alkaline cells.