I doubt putting second instance in a separate rack is likely to make much of a difference - it it does I say it’s just luck and a timing issue with the order things are processed.
In generally whenever I see issues with multiple instance my first reaction is a compatibility issue with Cantabile’s multi-core audio processing.
Try
- Setting Cantabile’s multi-core mode to “compatible” instead of “aggressive” (in Options -> Audio engine).
- Disabling multi-core all together (Audio Threads -> “1 (disabled)”).
If neither of these help then you really need to take it up with the plugin developer.
Also, there’s a history of weirdness with OPX and Presets - see here.
Brad