Interfacing BT and ADK boards

Hi, I am using two boards from Arduino, the BT and Mega ADK, to process some inputs on the ADK and output via bluetooth through the BT board. I want to transmit from one Tx pin on the Mega ADK to an input pin on the BT board. What I am asking is what pin on the BT would need this Tx input and I am missing anything in terms of communication between the two boards.

As an after thought I was curious if I could just take the serial Tx output on the ADK and just picking an I/O pin on the BT and just immediately outputting the data to the pin connected to the Bluetooth module