[Junk_Collector]

If you are an absolute nutcase, you could characterize a set of line stretchers and a multiplexer on a high end VNA then offset the inputs of the 4 channels on that UXR with them, take a capture and finally rebuild a 1TSamp/s signal out of the 4 results.

You have to have the 240V model of the scope to run all four channels at full rate (110GHz) though.

[cesaref]

The older Tektronix TDS540 series did this, but at much lower rates as was common in those days though. Internally there are differential feeds from the very beautiful hybrid ceramic input boards to 4 DACs, with some clever switching so that a single input can be sampled by all 4 DACs with a suitable offset to create 4x the sample rate when running with all 4 inputs.

The calibration procedure on the scope fiddles with the time alignment to get the different DACs correctly offset so that the combined signal is correct.

The hybrid ceramic input boards in their metal cases are a thing of beauty, fragile (don't ask how I know), but beautiful.

[Junk_Collector]

Yup, a lot of scopes actually did this internally and some still do. It's part of why some scopes lose half their BW when you go from 2 ports to 4 ports (some go the other direction and run multiple ports on one very fast ADC), they split the digitizers. It's just very very difficult to keep it working external to the box mainly because of line drift.

[jacquesm]

Take that Nyquist ;)