I have setup my reseller account with my own DNS,

webhost gave me the ip's to include at my registrar so I get :

ns1.mydomain.com
ns2.mydomain.com

Now, can I use the same ip's and at my registrar add these ip's to mydomain2.com the same way I did for mydomain.com and get also :

ns1.mydomain2.com
ns2.mydomain2.com

??

You can do this... but be sure to add the A entries to your dns zone as well. It's best to contact your host if you have any doubts or to make sure that you have set it up properly (assuming they will allow you to edit DNS zones or if they will do it for you).

Your best bet is to talk to your webhost to see what they feel is the best way to do the setup you are attempting. If they have a different setup on their end you might get some funny dns issues happening occasionally.