How can I allow internet services to both users from secondary ISP link when primary goes down?

If primary link goes down because of preemption secondary will come up and traffic will start passing to secondary ISP.

But i have a question here, hope you are clear with the concept of NAT, when traffic is traversing through primary NAT table is maintained on primary router but when primary goes down secondary will take its place , how NAT entries are replicated from primary to secondary.