Assume the PIX has three interfaces or elss trunking has to be enabled.
Run a routing protocol ospf between the PIX and the router
Configure a static default route with metric higher than 110 and point it to the fall back route.
below is a sample config
interface e0 outside security0
interface e1 outside2 security0
interface e2 inside security100
ip address inside 10.0.0.1 255.255.255.0
ip address outside 192.168.0.1 255.255.255.0
ip address outside2 172.16.0.1 255.255.255.0
route outside 0.0.0.0 0.0.0.0 routeb 250
router ospf 100
network 192.168.0.0 255.255.255.0 area 0
global (outside) 10 2.2.2.2
global (outside1) 15 3.3.3.3
nat (inside) 10 10.0.0.0 255.255.255.0
nat (inside) 15 10.0.0.0 255.255.255.0
static (inside,outside) 2.2.2.4 10.0.0.5 netmask 255.255.255.255
static (inside,outside1) 3.3.3.4 10.0.0.5 netmask 255.255.255.255