We are making some renovations to our house and one requires me to move our downstairs HVAC returns. They are under the stairs with one in the hall and one in the master. They are connected together.. for instance if i remove the filters you can see through from our hall to our master. There is 16" flexible ducting coming from the crawl to the bottom of a box that feeds both. While moving this i will need to extend the flexible duct about 5 feet but my question lies in what i want to do while doing this. I want to split these two returns so they can be in different places in the walls.. not back-to-back.. and also then having a bit more privacy for the master. I was thinking of putting a tee in the crawl and feed them each with their own 16" flex duct from the tee. Same size grills, etc. Is that okay? Since I am splitting them, could i go down to smaller size flexi duct? If that is okay, how would i go from 16" to two feeds of something smaller? It will be very hard to go all the way back to the air handler. Thoughts?