I was told by my Pediatric Ortho that 2-3% of people with Scoliosis who have a 40 Degree curve or more will develop Spondy. That doesn't mean that Scoliosis causes Spondy - it could be coincidence, or perhaps whatever 'caused' the scoliosis (like back trauma) is also causing the Spondy. All the same, I've heard from ALL my Ortho doctors that there IS a correlation.
You best bet is to ask your doctor - ever patient is different.
Best of luck dealing with your Spondy and Scoliosis - it's not fun, but it is doable.
Take care!