Different sampling rates for half and full channels on oscilloscope? How is it done?
Daniel from Keysight here - this is a very common behavior for this range of oscilloscope. Essentially, manufacturers (like us) build out acquisition boards to handle two channels at a time.

So, a four channel scope has two acquisition boards*. When you use two channels on the same board, the ADC has to cut its sampling in half. It spends 50% on one channel and 50% on the other.

If you'd like to dig in more, I highly recommend this talk given by one of our ADC designers:

https://www.youtube.com/playlist?list=PLzHyxysSubUmxGOMVpiKLxouweh2AAlG1 

**Acquisition systems would be more accurate, as it's often just one PCB. However, each "path" will have chips/ASICs/ADCs/processors that handle two signals per chip.*