You don’t need exactly the same cert for failover. You only need a valid certificate. You don’t even need the same cert for every entry in your load balancer. Client will pick a single IP address when resolved, then connect to it and will keep using that TLS connection for the whole session.

But you do need Let's Encrypt (or whatever ACME provider you use) to connect to the same server you are trying to set up the cert on. And they intentionally try to fetch the challenge response from multiple geographically distinct locations.