Ah, its between two different parts. Should have mentioned that initially.
So Rx to Tx, Tx to Rx, that is connecting input to output.
The program of the first Arduino is not written by me, and the serial port of the first Arduino already had its role which I don't want to touch. The part that I'm working on is the buttons on it (nine of them as a whole). Basically, I'm doing all these for fun and for learning what can and can't be done. Just in case you might be curious, I have a 3D printer driven by an Arduino board which has a LCD panel on it, I captured the LCD output with another Arduino board, replaced the buttons with my output pins, and resembled everything remotely on an Android phone via Bluetooth serial. All that had been done, and works nicely. I'm now double checking for any potential flaws. (for fun, cheers :P)