Upon setting up serial UART between two boards I can only monitor send or receive, not both, Why?
The setup you've shown won't work. UART and RS-485 are very different electrically.

UART uses two unidirectional pins (RX and TX).
RS-485 uses a single differential pair (commonly called wires A &amp; B) for bidirectional communication.

To make it work you would need to add RS-485 transceivers next to the ATmega and SAM, only then the 3 devices could share a single bus.

Alternatively you could use two USB to UART converters, hook up their RX lines (one to the TX of ATmega, the other's RX to the TX of the SAM).