MOD-UI is very pretty and quite cool but it does have some limitations. You can do what you describe using standard Zynthian layers, e.g.
- Add an Audio-FX layer, e.g JV/MDA Dynamics
- Highlight the layer in the layer screen and Bold press the Select switch to show the layer’s context menu
- Select “Audio Capture” which will show a list of the audio inputs available
- Select / deselect as required, e.g. select only the left input
- Within same layer context menu, select “Audio Output” which shows a list of audio destinations available including soundcard outputs, inputs to plugins, etc.
- Select / deselect as required, e.g. select only the left output
- Within same layer context menu, select “Add Audio-FX” to add more plugins to the effect chain
- Repeat for other channel, i.e. add effect layer, route inputs and outputs, add other effects
Cheers