"With fault tolerant architectures, server components are doubled up and cross-coupled in such a way as to present a single image to end users, with the two duplicate systems running in absolute lockstep."
That's a fair description of the way Stratus provides a fault-tolerant hardware platform. Tandem's approach is more software based, with keep alive messages between duplicate software instances allowing seamless takeover from a failed process. No lock-stepped CPUs are involved.
There are advantages and disadvantages to each. It's more difficult to port third party apps to a Tandem platform, but OTOH software is a much more frequent cause of system failures than is hardware.