Can I have two ReadyNas 316 (or another model) systems auto back up to one another?

If your intent is that the contents of the two NASes will be the exactly same, then Replicate is not your answer.  Though they call it "Replicate", it doesn't truly replicate; it's a backup and versioning system. Not that that's "bad", it's just not true replication.  The backup files do not go into the same directory structure on the backup as on the source.  If your primary system dies, you can buy a new one and restore from your backup.  But you can't rename the backup machine and have it take over from the primary.

And if you will be adding and deleting or changing files on both NASes, I don't think there's a good solution, either.  If you use RSync and set it to delete files on the target that have been deleted on the source, it's also going to delete any files you added to the target but not the source.  Likewise, if you delete files from the target or modify them there, it's going to put them back from the source.  So, you can only have one primary and one backup system, not a two-way cloning.  If you use something other than RSync, it won't delete files on the target that have been deleted on the source. 

I tried to figure out a way to use a combination of methods to produce cloning when files are being changed on both systems, and could't do it.  Robocopy via a Windows computer or some other third party sync method was the only way I could see to do it.