The original Tandem systems involved all processors on a redundant local bus. The system was later extended to allow processors to be networked remotely. The two machines acting as primary and backup for a transaction no longer needed to be in the same room. They did require a low-latency connection, though. This was all pre-Internet, so Tandem had their own protocols.

[1] http://www.hpl.hp.com/techreports/tandem/TR-86.2.pdf