Thanks for the welcome and for the reply.
Why would a second vCenter be required, and would only machine A and B be in the second vCenter? Or are you saying that machine B needs to be in it's own vCenter instance?
So far we've evaluated backup only solutions, such as Veeam Backup and Recovery and VMware Data Recovery, mainly because we already have licenses for those products and thought they might work. Turns out backup is too slow because of the amount of data we need to replicate (6TB). This is why I thought something that replicated changed blocks only would work better.
We're on a tight budget (this is a development environment, not production) which is what drew me to SRM--it's listed as something around $500 per VM. Due to the nature of our hosted environment, only one server type (Linux NFS) is not in a A/B pair. We're in the process of evaluating BRBD in case we can't find a commercial solution.