If you have two ISPs and you want to use the first one for outbound internet access for hosts on your inside network and use the second ISP for people on the internet to access your servers.
interface Ethernet0/0
nameif outside
security-level 0
ip address 1.1.1.1 255.255.255.0
!
interface Ethernet0/1
nameif backup
security-level 0
ip address 2.2.2.1 255.255.255.0
!
interface Ethernet0/2
nameif inside
security-level 100
ip address 192.168.1.1 255.255.255.0
!
interface Ethernet0/3
nameif dmz
security-level 50
ip address 172.16.1.1 255.255.255.0
global (outside) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0
static (dmz,backup) 2.2.2.4 172.16.1.2 netmask 255.255.255.255
route outside 0.0.0.0 0.0.0.0 1.1.1.3 1
route backup 0.0.0.0 0.0.0.0 2.2.2.2 2