just think of a 2 channel Amplifier as 2 separate Amplifiers which share a common Power Supply.
Likewise for a 4 Channel Amplifier.
With a single 2 Channel Amplifier; you can run Mono Bass through one Channel at the same time as Mono Mid/High through the other.
For Stereo you will need extra channels to split Left and Right Signals to your 'speakers respectively...
Back in the good ol' days, it was popular to run different sections through an Amp, Running bass and then something lighter through the other channel meant there wasn't such a big draw on the power supply.