dedicating 2nd ethernet port for backups.. how?

As long as the two interfaces of the two NAS are on the same IP range, and that this IP range is different from the normal IP range (otherwise you may have problems because the NAS doesn't know where to send the data), then they should be able to communicate. There was a time where you had to use a crossover cable to do it, nowadays the network cards do this job for you and any network cable should do the trick as long as it is a cat 5E or better (otherwise you won't have gigabit speed).
So the simplest way is to use static IP indeed. By default the interfaces would get APIPA addresses if no DHCP is found but this is not a best practice to use those (even though this should work). You can also set a DHCP server on one of the devices if you want but that's just not useful.

Once you get those IP set, you can use any direct backup method you want. This includes smb, FTP or rsync for example.

thank you, xeltros. Do you know if OS6 supports btrfs send/receive of snapshots? That could potentially be a MUCH faster and more efficient backup mechanism. (Actually, I'm going to have to do some searches to find out if OS6 supports any type of snapshot backups at all...)

The backup utility integrated in OS6 doesn't. I don't know if this is possible without using things like DRBD.
That said, you can have snapshots on both NAS, each one would do its own snapshots (which is actually better because if one snapshot gets corrupted on one device the other is not impacted). You would just have to set the configuration on both NAS for those.
Rsnapshot can be used via SSH, and the ReadyNAS replicate will use the direct link when possible and has the snapshot function I think (not sure)

OS6 has the ability to (via GUI) :
Take snapshots (configurable on each share under the label continuous protection)
To verify data checksum
To virus scan
To backup via an utility located in the backup tab

I would advise staying away from SSH if possible as Netgear may deny support depending on what you've done (And because their backup methods won't be affected by updates, whereas if you do things yourself you never know).

Gary,
Have you actually tried what you are proposing in practice?

Could you run a backup job, for example at midnight, or at an off peak time.?
Backups between 2 NAS on a LAN should be pretty quick for small data transfers.

Have you considered Replicate, which deals with incremental changes?

In theory, you could set the 2nd NIC of each NAS to static IP,
eg 192.168.x.210 and 192.168.x.211, same subnet mask 255.255.255.0 and try a backup job.

There are no KB articles exactly matching your proposal.

Static IP [http://kb.netgear.com/app/answers/detail/a_id/7001/url]
Sparc to OS6 [url:lj1labig]http://kb.netgear.com/app/answers/detail/a_id/7054

Basic OS6 Backup articles are linked off
http://kb.netgear.com/app/answers/detail/a_id/23420

Regards, Marto

garyd9 wrote:
...Actually, I'm going to have to do some searches to find out if OS6 supports any type of snapshot backups at all...

If you turn on user access to the snapshots, then they will be included in the backup unless you exclude it. That's generally not a good thing, since if you copy a single snapshot you end up doubling the storage needs, and usually there are a lot of snapshots.

btrfs itself puts the unchanged files into multiple folders (e.g. the snapshots and the main share) - so each snapshot appears to be a full copy of the share. This is a bit like a link, but it doesn't show up as a link. So if you back up or copy a snapshot, you are copying the complete share as it existed when the snapshot was made - not a delta.