[Hikikomori]

Yep, we also have two. Not a DNS expert but wouldn't half the clients (or their resolvers) stop working if one of your two providers are down? But you can recover by removing the provider that is down from your NS, assuming that the TTL is not too long.

[toast0]

Generally, recursive resolvers will retry queries multiple times and use all the NS records as needed.

You shouldn't have a broken NS record if you can avoid it, and it may add a little bit of delay, but things will still work.

Some recursive resolvers also keep statistics about authoritative servers, and when a domain had multiple NS records, the recursive resolver will prefer the option that has had the best response time in the recent past. That helps hide broken servers.

[zinekeller]

> Generally, recursive resolvers will retry queries multiple times and use all the NS records as needed.

IIRC some recursive resolvers ask all of them simultaneously and then just pass the first request received.