You can use as many DNS servers as you like, a domain name will still be resolved correctly. They will answer the same: the domain names registered IP address. The only difference might be how fast changes are propagated to different DNS servers.
Also note that whatever DNS server you use is irrelevant for users of your web page or whatever you want to publish under that domain name. Users have their own configuration which you have no influence on whatsoever.
The order of resolution, so which server is queried when depends on your setup. Most systems offer different means of prioritizing queries. In general: if some server fails to resolve the domain name, then other servers from your list will be queried.