Let's take a step back. If either firewall can handle all the traffic with room to spare, what exactly is the point of load balancing? Sure, you could do it, but with no benefit. It would just make troubleshooting that much more cumbersome.
Also, I don't understand your diagram. You said your ISP is putting a single router between you and the actual connections. So, in essence you're presented with a single connection. Is that not so?
Also, I don't understand your diagram. You said your ISP is putting a single router between you and the actual connections. So, in essence you're presented with a single connection. Is that not so?