If I understand you right, the UARTs are otherwise occupied, otherwise that would be a solution.

Pop the Rx2 and install P89C66x which has IIC hardware. Then interconnect is a breeze. Soft IIC only works well when nothing else is going on.

Another option would be to replace the Rx2 with a dual UART device and use RS-485.

Erik

Ps to find an appnote for 66x IIC use the LPC932 appnote, the changes are nominal.