It's only an issue if you wan't to use a mouse or a touchscreen or similar to control the modulation right?
If you have a controller with a joystick or X/Y pad, then you can just let one axis control one volume and the other axise control another volume.
This can be set globally using midi CC assignment.
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In Pigments it's also possible to for example assign multiple parameters to a Macro control.
The Remap functionality in the Combinators also give you the possibility to create different modulation curves.
So you can do more complex modulation.
This has to be programmed per preset though.
