I have a small powder room that doesn't have a register/vent so it gets a bit cold compared to the rest of the house. I could install a new duct line split off from the main but there is a wall/floor register right on the other side running in the wall cavity in a 2x4 wall. I'm wondering if I can just cut out the back of the rectangular duct and install a baffle to split the air to two registers and then fine tune with how open the registers are. Or is there a premade solution for this? I found some split return air ducts for equalizing are between rooms but not this specific setup
My parents' house has one place with vents that look exactly like that - the duct has two vents, one on each side, and there is a baffle inside such that you cannot look through the vent and see the other room. The baffle extends about 1-2 inches past the edge of the registers.
I spent some time trying to find a pre-made part (or even a picture of someone having done this), but I can't find one.
This was a common practice in very old houses especially when running heat ducts to a 2nd floor. The only problem is that the room needing the most heat may not heat properly. The fitting you are asking about were called double stack head duct fittings or a similar name and are still available. Myself, I would buy a 4" hole saw and drill a 4" hole in the plaster/drywall and through the back side of that supply duct. Then seal the newly cut hole and install a 4" plastic sewer fitting or similar fitting into that hole. You could go to a big box store to match a sewer fitting or vent fitting with a correctly sized hole saw. (hope this helps).