Submitting an extension paper of a paper that is currently in review for a considerable amount of time?
I wouldn’t submit the follow-up paper while the first is still under revision. For the moment I’d consider the time that you’ve waited as pretty normal. So stay calm and wait some more time. If the first paper eventually gets publish, fine, then proceed with the follow-up paper. Otherwise, I would consider the option to merge both versions to a more influential single version and submit this one (maybe even in a higher-ranked journal).