FabFilter User Forum
Linking Volcano's Filters
Thanks for the feature request. You're right, you can't link one MIDI controller to both filter cut-offs.
But what you can do, is press the ALT key while changing a filter with your mouase in the interactive filter dislay. Then, both filters are linked and you can record automation data for it!
Cheers,
Floris (FabFilter) — Jun 3, 2007
As far as I can tell, there is no way to link the two filters in Volcano. This would be a nice feature to have, especially when automating frequency with MIDI and trying to retain the curve of both filters.