Multiple failover for load-balancing?

I have 6 WAN modem/routers in load balancing on an EdgeRouter ER-8, giving internat access to our internal LAN. It works quite nicely now that I have set them so that their gateway adresses do not overlap, and I'm delighted by the router's performance.

Some websites haves issues with multiple IPv4 sources for the same session. I solved that with an additional load-balance group B having two interfaces, with one declared failover-only; and assigning it that particular traffic:

load-balance {
    group B {
        interface eth2 { }
        interface eth4 { failover-only }
    }
    group G {
        interface eth1 { failover-only }
        interface eth2 { }
        interface eth3 { }
        interface eth4 { }
        interface eth5 { }
        interface eth6 { }
    }
}
/*..*/
firewall {
    /*..*/
    modify balance {
        /*..*/
        rule 500 {
            action modify
            description online.net
            destination { address 62.210.0.0/17 }
            modify { lb-group B }
        }
        rule 900 {
            action modify
            description "general traffic"
            modify { lb-group G }
        }
    }
/*..*/
}

That works! However, in the (admitedly unlikely) scenario that eth2 and eth4 both go down, I would be toast. It would be nice to have a way to give more interfaces to group B, so that the first interface that is not down (in some order that I decide) gets used. I tried multiple failover-only, but that's refused. Any idea?