Slab House, when using main bathroom the 2nd bathroom gets humid as well

I’ve noticed that whenever someone uses our main bath to shower, our 2nd bath gets humid and smells like the shampoo & body wash that was used in the 1st bathroom. It’s not subtle either. It seems as if someone showered in the 2nd bathroom at the same exact time. Right after we moved in, the main sewer line was backing up thru the 2nd bathroom and kitchen sink. Do you think this could be a sign we are going to have a sewer line backup again? Or could it be something else?