Redundancy IP address for website

We are hosting one website  with one public IP address and Customer added that Public address on the Domain  server from go- daddy . Now customer wants another public IP address from different ISP and he can’t add two Public IP address to his Go-daddy domain .   Please advise any other possibilities to add two Public IP address from different ISP mapped to one Private IP address and Linked to  website on the outside domain . is there like any Virtual IP address concept will work in this situation ? if yes please advise me how.   Many thanks