Are you wanting the same public IP assigned to a different interface? Do you have a static address assigned now? If so, what you want to do isn't possible that I know of. You won't be able to assign the address to the other interface because the router will tell you that the "address overlaps interface xx."

You can have different addresses assigned to your interfaces (they have to be in different subnets) and have routes force all traffic out one interface, and run ip sla to track that interface. If the interface goes down, all of your traffic can be pushed out of the other interface.

You can also just have two interfaces with two different addresses and run floating routes in your config.

ip route 0.0.0.0 0.0.0.0 fa0/0

ip route 0.0.0.0 0.0.0.0 250 fa0/0 254

OR

You can track the interface:

track 1 int fa0/0 line-protocol

ip route 0.0.0.0 0.0.0.0 fa0/1 track 1

I'm not 100% sure if the last one will work because I've never tried it. I'm not sure if the track line in the route statement will ONLY work if the tracked item goes down.

HTH,

John

What is the concern, device/service connecting the 2 routers will go down? i.e. redundant switch in between the 2 routers or WAN links (serial)?

There are alternatives in achieving that failover if you give us more info on how the two routers is connected.