How do I connect 2 boards together?

wvmarle:
If you use the Tx/Rx for this, you lose your Serial monitor for debugging.

Why do they recommend serial over I2C?

I'd go for I2C for simplicity and speed. Can't think of any advantages of UART serial over I2C.

They haven't said why they used that option, the Instructables are quite useless tbh, they skip over alot of important information on connections