Pin access to UART on APM 1.0 and APM 2.5

I have 2 APM units that I am using for a project with custom software, and I want to be able to communicate between them, while still having access to all the sensors on the board. Serial is my first choice, since it is simple, fast, and reasonably reliable over wires. I also need to be able to use one XBee to communicate to another Arduino elsewhere (from either the APM 1.0 or the APM 2.5). Is there an unused UART port on both the APM 1.0 and the APM 2.5? And if so, is there an Arduino pin -> APM pin mapping available? I wasn’t able to find one during my searches.

Any other suggestions for wired communication between the boards?

Yes, there is UART2 on the right side of the APM2.5 board. If you look at the underside of the board you should see some labels there. We generally don’t use UART2 on the APM2. UART0 cannot be used because that’s used for the USB connection.

 I'm not sure about the APM1.