So here's what happened. My queen got captured in sub-to-sub combat, and returned to the nearest enemy outpost. My princess became queen, and was moments away from capturing that outpost and getting my old queen back. My enemy released the old queen, sending it back towards my base. My attacking sub hit the returning queen, getting her back under my control. For just this moment, I had two queens. As soon as my attacking sub landed, that queen reverted back to a princess.
I'm surprised it was the original queen that stayed queen, and that they didn't swap places. I thought after the princess became queen, she would stay queen, and the second one after her (who happens to be the original), would be the one to become a princess. Maybe it's based on whichever specialist is older?
As far as specialist hires, I don't think it's as closely tied to the queen as it seems. Getting a queen captured while you have a princess doesn't do anything to change the specialist hire times or choices, so it's more like the queen lets you access the specialist hire that's separate from her. If you could somehow have two queens, it might let you choose where to hire your specialist, but wouldn't give you an extra one.