the best configuration depends really on your situation, you can run both nameservers on the same machine, but it means that as the nameserver crashes the sites would not be available anymore (there would be not alternative location to check for the records anymore).
Although if your server crashes; it also means that your sites are not reachable, but that would be anyway the case as the webserver is down in such a case as well. Only if you use i.e. a backup MX server (mail server) it would have been a good thing your nameserver kept reachable to mail could have been send to a secondary location, but we dont see many who really do have a secondary mx location.
If the outages would be short; and you can restore your service quickly it mostly would run fine in a single nameserver setup; granted it is not optimal and if you have the resources to host a secondary nameserver; it certainly does not harm.
It is always a question of what you try to accomplish and what budget you have to do it.