You should rather use ONE domain and set of nameservers for all your domains, so make ns1/2.xxx.com the nameservers for your yyy.com.
But I was told by someone (at WHT forum) that it is possible to have multiple name servers. So just checking that out.
It sure is, but why exactly would you want that? It would just make it much more complicated to manage, since you can’t use Virtualmin’s usual features of assigning nameservers to domains then. Also you’d need glue records for each of your domains at your registrar then, if all nameservers are in the same zone they serve.
I already created name server at my registrar and it still works fine.