Yes, you can. Ring Motability and tell them you do not want to use your entitlement on your Motability vehicle. You can then sort the tax yourself and use your Certificate of Entitlement on your private, more expensive to tax car.
I’ve done this with my Motability EV (£0 tax) and my personal £290 tax car. I’m responsible for doing the paper exercise ‘taxing’ the free to tax EV and use my entitlement on the private £290 car.