You're running into an unfortunate shortcoming in the operation of the BSD-derived networking stack used by IronPort. There is only one default gateway, and it is only reachable via the interface that's on the same IP subnet. You cannot have separate default routes on each interface. The interface used to transmit a packet is chosen based solely on a routing decision. If the destination address is on the same subnet as a local interface, then that interface is used. Otherwise, the routing table is consulted to find the closest match for the destination address. In a typical setup for an end-node system (i.e. not a router), the only thing in the routing table is the default route. What's notably absent here is what is not used for picking the interface: the source address on the packet. It is easy to think naively that if the source address of the outgoing packet matches the address on an interface, then that interface would be used to transmit the packet. But that's not how it works. Only the destination matters, and the interface that's chosen is the one that's closest to the destination.
This leads to all kinds of headaches, especially if, like us, your networking guys enforce uRPF. In this case, transmitting the packet on an interface other than the one whose address matches the source address of the packet will result in the router dropping the packet as having been spoofed. I went round and round with this a few years ago trying to have the management port on a separate subnet and yet still be reachable from the rest of our campus network. It was only reachable on its own subnet, and uRPF turned out to be the problem. Traffic from my workstation to the management port took the right path, but the return flow had to follow the default route and thus was transmitted on the Data 1 interface instead, resulting in a uRPF violation.
Without seeing a diagram of your network and what you're trying to do, I can't be certain that this is the problem. But it certainly matches the symptoms I saw. If I'm guessing correctly about what's going on, then I'm afraid you can't get there from here.