As I posted earlier in this thread the normal way to setup load balancing and/or failover on multiple ethernet interfaces on the same subnet is to use the bonding driver. You will end up with one IP address for the bond interface and it will accept traffic from any of the bonded interfaces. Traffic out depends on how you set up bonding but it can be load balanced and/or failover.
A product that I developed code for commercially I used just that setup with CentOS (open source Linux equivalent to RedHat). We allowed the operator to group as many interfaces as they desired into one logical (bonded) interface for the purposes of reliability and greater throughput. It works well.
I haven’t done bonding with ZS but I do see a bond interface option under the network setup. You might want to give that a try.
Edit: Some of the bonding options require both ends of the connection be configured appropriately. You may need a commercial grade switch to properly support all bonding modes. But simple failover and at least one load balance mode are, if I recall correctly, compatible with consumer grade Ethernet switches.