New here and new to Yamaha instruments… SO i’ve been using a Kurzweil K2600X along with a K2000 just for the keyboard itself, making glissando a bit easier… I recently got a Motif rack ES and soon after, a CS1X.  With the CS1X, I can control basic sound parameters such as cutoff, res, att, dec, etc. of the Motif rack i’m using it to control.  I want to be able to run that signal (of the coupled yamahas) into the K2600, as it has beautifully weighted keys and feels like a dream. I have been killing my brain trying to figure this out and if anyone can help I would be greatly appreciative.  Thanks for any help!

You need to study the MIDI implementation documents for the Kurzweil to see what parameters can be controlled by MIDI CCs (continuous controllers).

The reason the CS1x can control the Motif is because Yamaha is consistent in the CC assignments across instruments.