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).

In what field are you working?

I would suggest to proceed with caution because of several factors. If you submit your new work and gets published faster, there is a risk of rejection of your first paper because it is not novel enough, etc. I have heard of a case in physics in which the review process took ~1 year, and it was eventually rejected because the same authors during that year generated data with better resolution, and thus their original work was not worth as much as it was.

Another important point to remember is that you can't plagiarize yourself. This means that if you add appendixes with copy/paste of your own paper, that does not count as plagiarism. However, journal editors may disagree. Many journals have automated software, and if it gets marked for not-to-publish, editors won't spend a second more on it. Again, proceed with care, because your first paper can be locked out because of your follow up paper.

If you have the option to submit to different places, I would ask the first journal to withdraw the first paper from the revision process, and submit it somewhere else (but again, you are the only one who can judge whether it is smart for your career). Sometimes just asking it, makes the editors speed up the review process.

Finally, you can consider publishing your first paper to a pre-print server, and cite it in your new paper. If it happens that your first paper gets approved before publishing your second one, you can update the references. And if not, both reviewers and readers have access to the original work if they want to follow your procedure.

I think that it is a tricky situation, in which a lot of what is wrong with the review process starts to surface. In some areas, reviewers block the publishing of papers because they are working on the same area and they want to be published first. You can always pressure the editor to speed up the process, perhaps finding new reviewers, or if you say you are going to submit elsewhere and your paper is worth it, editors will speed up the process. Remember that publishing is, in almost all cases, a business, and losing a paper is losing money.