I dont think rsync would work since server2 would be updating named.conf also.....so server1 nameb.conf would get overwritten with server2 and then he loses all those domains....You should definetly have ns1 on server1 and ns2 on server2 incase on of them goes down you have the backup. Someone has a script out there to tie in with plesk im sure...
I think the best way to do this is to set up server 1 to run a secondary DNS service for each of the domains on server 2. Plesk has no way to create a secondary DNS service, but you can do this from the shell, or write a simple script (I wrote one for myself in a few minutes). But, you tell the registrar that the nameservers for those domains are the ones on server 1. It doesn't matter how the records get to server 1, only that they are there. This is sometimes referred to as a blind secondary - it's listed as primary, but is in fact a slave to the records of another server.
This way, any changes you make to the DNS records in server 2 send notifies to server 1, which picks up the changes automatically, using the whole primary / secondary DNS scheme.
I believe also, with a little thought and some time, you could get server 2 to build a list of any recent domains added to it, and have server 1 retrieve that data and create it's own secondary zone config as a cron job.