"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.

I believe that the observation "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." is not quite right.

Tandem does use lock-stepped processors; a pair forms a single 'self-checked' processor. Each of the primary and secondary processes (forming a duplicate software unit with keep alive messages etc ) runs on a distinct 'self-checked' processor.