From what I've seen in the database, it uses the IP address to identify nodes. It is also used for polling. I don't think you're going to be able to find a way to have the same IP address, with different community strings. The only way I can see a network management product could do it would be if it was set up for full multi-tenancy, and so allowed overlapping addresses. But that doesn't really solve your problem.
Are you sure that the firewalls don't have separate management, non-clustered IP addresses?
Out of interest, what model firewalls are these? I haven't come across that particular setup with any of the firewalls I've worked with so far, and I'd like to avoid it in future.
