Update 2: NAS2 is now successfully connected to ZenServ2. I made several changes at once, so I'm not sure which one fixed the problem. I will go over them one by one in the hopes that it might help someone in the future:
1. My realm is something like local.domainname.com. I actually own domanname.com on the interwebs, and I had forgotten that for that host's DNS records I had defined local.domainname.com with another IP (an A Record). I'm not sure if that would make any difference as I was telling NAS2 to use ZenServ2 as its DNS server, and I would assume that ZenServ2's DNS would override any external DNS. Anyway, I deleted the A Record on my internet name server.
2. On both ZenServ1 and ZenServ2 I defined the OTHER Domain Controller under the webadmin -> DNS -> Domains. That includes adding the IP of the OTHER server, adding the Hostname of the OTHER server (and then the IP again), and finally adding the OTHER server again under Name Servers. I can't find the page on the Internet anymore, but I did this because I remember following a guide for setting up two Zentyal Servers as PDC and ADC back in the 3.0 days and it instructed me to do this. Once again, I'm not sure if this is necessary, pointless, or harmful in 3.5.
3. On my Gateway Servers, which we can call Gate1 and Gate2 for Network1 and Network2 respectively, and which were formerly my primary DNS servers as well, I made sure to define local.domainname.com as the IP for ZenServ1 (on Gate1) and ZenServ2 (on Gate2). Again, I'm not sure why this would make a difference since I was telling NAS2 to use ZenServ2 as its DNS server; however, Gate2 had been its DNS server before.
4. I also rebooted NAS2 after making all changes. Even after rebooting, it still wouldn't work (a new error now: told me "Windows Domain cannot be found"). I had to load the Directory Service tab, then change to another tab, and then go back to the Directory Service tab again before it worked. I actually just finished making a change to both ZenServ1 and ZenServ2 and it looks like NAS2 lost its connection to the domain. I actually had to reboot it and then change tabs SEVERAL TIMES before it finally connected (same error about "Windows domain cannot be found"). Honestly, at this point I'm not sure if the changing of tabs was doing anything, or if I just had to wait a certain amount of time for "something"...