It seems that you have 2 ideas in mind- fading the speakers(ie:changing the volume level) or switching different pairs of speakers on and off using a switch.
I think that all of this would be easier if you used a 4 channel amp- and will also give you better sound. Most of the time, motorcyle amps are cheap and small and work very well.
Using switches would be the easiest way- but the speakers would only be on or off, no middle ground.
The (more expensive) way to fade the sound would be to use either a remotly mounted gain control knob for the amp, or to use impedance matching volume controls- niether is very cheap.