And in this fictional world it's totally possible that the inhabitants always believed the 2nd moon to be a moon, but may later find out that it is actually something else.. Like a planet, (or meteor).
What Cairdeas said makes sense.
In the lore they're only called moons because the people living on the planet believe them to be moons. They could be wrong. Even with the lore this would make sense. I hope I explained this well enough.
As an example.. we've called Pluto a planet for so long. Now we're being told "oops, we were wrong, it's actually something else." It's not unrealistic to believe that the same could happen in a fictional world.