That sounds pretty cool actually. Assuming two servers had equal weight in MX records, such that mail had a 50/50 chance of going to either, you could unify it via fetchmail, or similar, if you polled both backends directly.

Usually it is a pain if you have mail going to more than one host - ie. no shared storage amongst all the hosts that receive mails, but if you were looking for redundancy and didn't mind the "manual fixup" this would be almost trivial to setup with 2+ VPS from different providers.