Hi buddy..
How can we link up 2 x unit’s RB750 port ETh1 to become we can have 8 port to be use (each from port 2 to port 5).
If you need more switched ports just buy a switch.
If you need more routed ports just connect them with a /30 between them and run a routing protocol or static routes for traffic between them, and a default route to the Internet or the other router.