IMHO your "backup" is not very efficient, as every target node has to be large enough to handle all traffic. We use 3 backend nodes, they run in parallel. They are sized for 2 to handle all the traffic, one can fail, they would be only "half the size" of your nodes. Of course this depends on your desired availability and budget.
If for session reasons your clients need to connect to the same backend again, you can use sticky sessions.