To accomplish this, get a Midi merge box. Connect midi guitar out to midi in1 on midi merge box.
Connect K8 to midi in2 on midi merge box. Next, connect the midi out of merge box to midi in2/kbd of SD9/Sd90 (only port that recognizes chords unless you set midi in1 to keyboard in Menu-Midi-).
Now the K8 transmits by default on channel 2 (chords) so set your guitar to transmit on channel 1 (to play lead voices like piano, sax ...etc).
Now in SD9/sd90, go to menu - midi (Standard should be selected /yellow) - Arranger Keyboard - Chan Rx and set left =2, right =1, Global =0 and save.
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Thanks,