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The SNAP-PAC-R1 (where R stands for Rack) is both a controller and a brain in the same rack-mounted hardware, which sits on the same rack as your (up to) 16 I/O modules.
Re: your question about the R1 Rack A as the controller for both Rack A & B -- yes, but... The strategy in A's R1 could have an I/O Unit for itself (e.g. IP address 127.0.0.1) and another for the R1 on rack B. However, the strategy's wouldn't be EXACTLY the same, since they'd have different IP addresses for the "other" controller, and I'm not sure why you'd want to do that.
When you talk about "failure" -- what kind do you anticipate? Loss of power? Communication? We do have some options for "redundancy" but that's usually for a complete duplication of the whole system. And since both the controller AND the brain are the same hardware, whatever might go wrong with one (power, etc.) would usually knock out the other.
Can you tell us a little more about your application and concerns re: possible failures?