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If both circuits go to the same firewall then you might be able to create two NATs to the same Gateway VIP.

They are both connected to the same firewall.

Even if not connected to the same firewall, you could have 1 public IP per firewall, and use basic DNS load balancing, by basic I mean round robin. We've got this setup in place, that's how we took off load from an overloaded Firewall. No need of GSLB except if you need advanced rules.