Granted we've been given limited information, but based solely on that information, it's not clear to me why a dual WAN router is necessary, or what advantages it would offer.
Typically a dual WAN router (at least for consumers) would be used to allow multiple connections to the internet, then hide that fact behind a single network. And that's why dual WAN routers typically offer load balancing, failover protection, etc. It only makes sense given the fact that users don’t particularly care which ISP is used, to them it doesn’t make a difference.
But you’re describing something much different. For YOU, it DOES matter. In fact, each WAN represents a different network for completely different purposes. And while you can use a dual WAN for these purposes, the question becomes, why? It’s not (as I described above) as if you can obscure the network differences (e.g., 192.168.1.x of the circuit system, vs. internet addresses) behind the dual WAN router. Every client behind the dual WAN still has to appropriately address the right network. But you can do that (and perhaps already are) using a single WAN router, patching the second network to its LAN, and using static routing to route clients of one network to the other.
My point is, it’s not clear to me that using a dual WAN really solves anything based on what’s you’ve described so far. As far as I can see, a dual WAN will merely push the routing decision from inside the LAN to outside the LAN. What’s the advantage? If anything, it offers a serious disadvantage; like the internet, users on the circuit system are blocked from accessing the other network by the WAN’s firewall!
Understand, I'm not saying the case can't be made, it's just that I haven't heard it as yet.