NGINX Plus Cluster - NICs

Your questions are too general as depending on the network environment what are different best practices . It is good to know that by default the NGINX changes the source ip on the server side so the traffic will return to the nginx as ip-transparency is not enabled by default so single interface shouldn't have an issue and with XFF header inserted by NGINX the servers will see the real client ip.

https://www.nginx.com/blog/ip-transparency-direct-server-return-nginx-plus-transparent-proxy/

Better first check if NGINX has public documents as HA file sync you could have just searched and found the link below as NGINX has great available documentation for every feature and if something is not clear or you see an issue then it is a best practice to check with the community when you have checked the NGINX documentation:

https://docs.nginx.com/nginx/admin-guide/high-availability/configuration-sharing/

For proxy and load balancing better check TCP/UDP load balancing and HTTP load balancing:

https://docs.nginx.com/nginx/admin-guide/load-balancer/tcp-udp-load-balancer/

https://docs.nginx.com/nginx/admin-guide/load-balancer/http-load-balancer/

I would also suggest to go through the NGINX training first, so you can have a better view on the product and this will help you to ask more precise questions:

https://www.nginx.com/learn/nginx-training/

https://www.nginx.com/c/microservices-march-2022-kubernetes-networking-agenda