DISTRHO https://kx.studio/Plugins:OneKnob#AB-OutputSelector
1 Pedalboard using it

OneKnob A/B Output Selector

Plugin screenshot
Utility

Control Default Min Max
A/B Select 0.00% -100.00% 100.00%
Mode 0 0 1
https://kx.studio/Plugins:OneKnob#AB-OutputSelector v.1:2.0-1