Search google for "Agilent G-Rex". A few documents leaked with my code name.
[G-Rex means Godzilla, but easier to type.]
There are 4 A/Ds sampling at 40Ga/s each. In front of each A/D is a 33GHz pre-amplifier. The best way to describe a pre-amplifier for an A/D is that it conditions the incoming signal for the very sensitive sampling hardware. (It is also where stuff like the attenuators and edge triggering flip-flops are located.) If you download the Infiniium 90000-X datasheet, you can see a picture of the front-end MCM (multi-chip module).
So that gives them 4 channels around 16GHz and then 2 channels of 33GHz by interleaving 2 of the A/Ds. That's all real-time and pretty straight forward.
To get to 60GHz+ they use frequency interleaving in front of the pre-amplifiers. By down-mixing signal content from 33 to 60GHz down to a range of of 0-33GHz they can sample "high frequency" content with their lower frequency A/D. Once sampled, software DSPs re-mix that content back into the 33 to 60GHz range. Then DSPs (which is probably just software running on the on-board PC) "stitch" together the native 0-33GHz content with the 33-60GHz content to create a 60GHz+ bandwidth single.
The stitching technique was first used by LeCroy a couple of years ago to boost their 6GHz real-time scopes to 13GHz.