I am finishing out my basement and need some help on the duct work to the bathroom. The bathroom in the basement is right under the bathroom on the main floor. The bathroom on the main floor is only 20 sqft. I would like to use the same duct work to split the two rooms. I know this is not ideal but because they are both really small bathrooms (the basement bathroom is 39 sqft) and not far off the main trunk, can I use what is already in place or do I have to run more duct work. The picture shows the duct work going up into the main level bathroom. All I want to do is cut off the bottom of it and vent it into the bathroom in the basement.