Registrar here. This may be a requirement of some registrars but not a requirement of all registrars. And in any case you can fake this (see warning) by simply using the same IP address with a different host name or by using a different IP address with a made up IP address.
Warning: Obviously you should have two valid DNS servers no doubt. I am simply pointing out that there are work arounds to this especially if you are trying to get up and running and won't have the 2nd dns operational for a short period of time you can at least get started with DNS that should work (since it only takes one and assuming that one is working you will be serving up records.)