so, you still have 1 router with 1 serial and 1 ethernet.
You're going to attach 2 firewall to the ethernet LAN and have different internal address behing the firewall. Is it correct ?
If so, you can just use static routes.
On the router enter the following commands :
ip route 192.168.1.0 255.255.255.0 pix-1-ip-address
ip route 192.168.2.0 255.255.255.0 pix-2-ip-address