Mirror is for redundant purposes (i think) not for two independent server alocating diferent websites.
Yes, thats correct.
You can create primary and secondary dns that gets synced automatically like this:
1) Create a new primary zone on the ns1 server, this zone has one ns recod for the ns1 server and a second ns record for the ns2 server plus a A-record for ns1 that points to the ip address of the first server and a A-record for ns2 which points to the IP address of the second server. In the field "Allow zone transfers to
these IPs (comma separated list)" of this zone, add the IP address of the secondary ns server. In the first step, we created the full primary dns record.
2) In this step, wer create the record for the ns2. Go to DNS > Secondary DNS > Secondary zones and click on the add button. Enter the zone name in the zone field (the zone is the same that we created in 1) and in the ns field you add the IP address of the primary dns server (your first server) and click on save. Bind will now transfer the zone data automatically between the servers and will get updates from the primary server automatically.