Hi Yang,
sure, you can "just" use the UART5 including the TX pin on PC12 (connector layout was just made for convenient usage of two spektrum satellites which don't use the TX pin).
If you use UART5 as a "normal" uart (e.g. for telemetry/datalink) both TX and RX are enabled by default.
You simply define USE_UART5=1 (which is automatically handled for subsystems like telemetry).
USE_UART5_TX=0 or USE_UART5_RX=0 would explicitly disable TX or RX respectively.
The bigger question you have to ask yourself is what kind of format/protocol you want for the communication to the other board...
Cheers, Felix