Hey there! It's likely that your Dean has an undersaddle piezo style pickup. Piezos pick up the pressure from the strings on the saddle, rather than the vibrations of the entire body.
In order to hear your percussive playing more clearly using TWA, you'll need a contact pickup like the K&K Pure Mini that will pick up vibrations from the body of the guitar.
Some percussive players use mic or mic blend systems, but mic only systems will not work with ToneWoodAmp.
For sake of avoiding feedback, I'd suggest a contact pickup as opposed to a mic blend system.
I'd reach out to your local luthier for wiring options. I'm sure you could have them wired to separate output jacks or potentially the same output jack.
Otherwise, you may be able to simply leave the undersaddle piezo in place, keep it disconnected and run the K&K to a single output jack where it already exists.