Ray suggested below that there is more than one way to skin a cat. I will add another possible option to consider.
A two unit condo is a really stupid idea in the first place. It basically requires unanimous approval because the only other voting option is a tie. This property would have probably been better suited for a townhouse type of fee ownership, but that's not workable with the units being one on top of the other.
My solution is perhaps a bit complicated, but I would look into the possibility of simultaneously dissolving the condo association, create a tenant in common situation with the other owner, and then buying out that other owner's interest. You would basically be converting the property from a condo to a duplex. This obviously would require that you consult an attorney to effectuate that change.
I see several downsides doing that, however. First, you'd likely need to refinance your existing mortgage, and the existence of that mortgage might defeat my plan. Second, the total value of the building as a duplex is probably significantly less than the value as two condo units (although that's resale value, not rental value). Third, the legal fees would likely be significant. Forth, I'm not sure even what state we're dealing with here. If it's not Washington there could be other issues, or there might be issues doing that under Seattle's ordinances if this really is in the 98103.
If you really want to expand your investment in rental properties, it might be best to look elsewhere. Don't get too focused on this property just because you're familiar with it. And again, if you do want to explore this possibility, you will need to consult an attorney. That is not a do it yourself project. It's possible you might incur attorney fees and find out you cannot accomplish the task.
That brings up another option. Sell your interest in this condo and buy a duplex. Of course you'd need to look into your financing options for doing that too.