If both of your uC's can talk to a PC then they will not be able to talk to each other.
You need a
Null Modem Cable. As I said the sort version is that TX and RX wires (pin 2 and 3) are crossed.
This sounds stupid but: Think of it like this. You have two pipes.
Your friend speaks into one and listens to the other.
You speak into the one he listens to, and listen to the one he speaks in.
If you get it wrong you are both speaking into the same pipe and listening on the other (that no one is speaking into). It is a good chance that is what you are doing.