A special mini amp channel switcher/FX loop pedal (When you actually only use 1 of the 2 functions and the other is always on or off) Details in comments. : r/diypedals
How To Switch Amp Channels And Guitar Footswitch – FuelRocks
The GA-FC: Unlocking Performance Potential with Your Amp
Amazon.com: Donner Bass Guitar Amp Bundle with ABY Pedal AB Switch Line Selector Guitar Signal Splitter, 30W Electric Bass Combo Amplifier with Onboard Clean and Distortion Overdrive : Musical Instruments
1CH Looper With Simultaneous Amp Channel Switcher Plus Polarity Switch