You can have two artificial discs at adjacent levels. There are some insurance and technical problems however. Some insurance companies will not pay for an artificial disc and most will not pay for two.
The artificial disc works best for nerve pain (radiculopathy) without severe degenerative changes of the disc. If the disc is highly degenerative or if there is facet disease associated (degenerative spondylolisthesis), the artificial disc will generally not be effective.
Be careful regarding recommendations for artificial disc implantation. I have consulted with a number of patients on this board who have had these discs implanted for neck pain and have not had significant relief. The good news is that these discs can be explanted and converted into a fusion without much difficulty. That does require another surgery however.
Dr. Corenman