In the documentation for the Keysight DSOX1204G, it has the interesting note for the sampling rate of:
2 GSa/s (half channels) 1 GSa/s (all channels)
So, if I use all 4 channels on the scope, I get GSa/s, and if I use only two channels I get 2 GSa/s. I would assume at the core of the scope there's some low resolution ADC with a convert line that is tied to a 1GHz signal. For the 2 GSa/s are they just tying two ADCs together at the inputs but putting the convert lines 180 degrees out of phase, or is it something more interesting?