Although we are taught that two queens can’t survive in one hive, it happens frequently. It occurs most often when a supersedure cell hatches while the original queen is still alive. The virgin daughter hatches, mates, and begins to lay eggs right alongside her mother. This is usually a temporary situation, but it can persist for weeks or even months.
Based on my own experience, I think it happens more frequently than we realize. We often search for the queen and then quit looking once we find her—assuming there is only one. With that assumption, it is easy to miss the second one.
The photos below came from a hive getting ready to swarm. Many swarm cells were lined up on the combs and some had already hatched. It’s possible that one of these is a newly-hatched virgin. The more yellow of the two (the first photo) was both smaller and quicker, signs of a possible virgin. Although a hive usually swarms before the virgins hatch, cold and rainy weather may have kept the swarm from leaving on time.