There is a world of difference between frequency-skipping (used in radio links, for example, as a security measure inter alia) and mixing frequencies using local oscillators, which allows you to process the signals at a lower sample rate, and do other advantageous things with them. There is also a world of difference between two channels and two frequencies. We handle scores of frequencies ranging from 8 Hz to 500 kHz, for example, with the same instrument, many of them simultaneously such as when seeking passively for signals of opportunity within a wide band of possible frequencies.

A