Normally you'd use the 2nd link as a backup - to a different switch for example. Or in a mesh you can use it to connect ethernet devices via the wifi bridge/mesh.
For splitting the network traffic - In the device configuration->optional settings->interface and Network settings, you'll want to put both eth0 and eth1 as Backhaul. Then in Advanced port settings you'll want to sepcify the allowed and native vlans for each network.
thanks for your reply.. I don't want to use another port as backup.. I want to use it to connect to another Separate backbone network??
My requirement is very clear:
1. NIC1- Connect to Network (Enterprise) and access Enterprise network via the SSID mapped to NIC1
2. NIC2- Connect to Guest Network and access Guest Network via SSID mapped to NIC 2
Network for Enterprise and Guest would be completely separately (Physically also)