Multiple default routes over a single interface?

Can you have multiple default routes with redhat 7.3?

I have a number of servers behind a small firewall (pix 500). I am changing
the external IP's and I have another small firewall(pix 501) with the new
IP's. 

With a single default route, I can use either the old IP's or the new ones,
depending on the default route I configure.

With multiple default routes, packets can make the round trip to the server
through the old ip and firewall fine. 

If I try the new IP, it makes it to the server, but the return packet gets
routed to the old firewall and gets stuck. Probably because the internal IP
on the old firewall is lower than the one on the new one.

Using OLD IP
source------>[old firewall/old IP]\<--------------|
                                   \              |
                                    \--->[target]->
                                    /
                                  /
             [new firewall/new IP]



Using NEW IP
             [old firewall/old IP]\<--------------|
                                   \              |
                                    \--->[target]->
                                    /
                                  /
source------>[new firewall/new IP]

Thanks,

Drew
I