This is more of a design question, but it seemed most applicable in here.

Is is possible to have two bordermanager servers and have each of them NAT a public ip address to the same internal ip address? I know that this is physically possible. The issue becomes where your default route is set. You can only default to one (I have tried multiple without success), so responses will only work through the one that is set as the default route.

Does this issue go away with the mail proxy? I can setup external DNS to have both ip addresses and configure each mail proxy for their ip address. Both private interfaces of the proxies are reachable on the local subnet.