Why would you make a receivedChars2 on the Uno? The Uno is independent of the Mega so it does not know anything about the code on the Mega so you can call it receivedChars as well (receivedChars2 would indicate to most programmers that there is also another receivedChars like receivedChars1.
Or are you using receivedChars already for something else on the Uno (e.g. comms with the gsm shield)? If so, it's time to start thinking about names that make more sense.