To judge from the description, your question sounds like a DNS question. What to do when the ISP removes the domain reference for domian xyz.com? This is essential stuff – so setup an alert so you remember to pay the bill, and confirm that it is payed.
You can have 2 domains registered in the same DNS pointing to the same IP-addresses – no problem there. But you cannot DNS wise configure automatic failover between the 2 (when you forget to pay the bill). People browsing your webpage or sending you email will have to know the other domain name (loosing your dns records is like loosing the street address).
You can control the TTL value for your dns records but this can be overruled by clients and other DNS servers.
So keeping your hostmaster happy – that’s what you want to do.