I want to add a utility sink to a garage and ideally use a 1.5" drain line that was initially installed to capture the regen water from a softener. The softener is located in a machine room and drains, via 0.5" PEX, into the open end of an exposed 1.5" ABS drain pipe that runs in vertical orientation along the machine room wall. That section of pipe is p-trapped at the bottom and connects via a sanitary tee to a dedicated drain/vent line inside the wall separating the machine room and garage. As luck would have it, the perfect place for me to tee into this drain line, from the garage side, is just below where the softener waste pipe tees in. (Orange arrow in schematic). Can I stack sanitary tees and share this vent/drain line? Is there a functional and/or code issue with this? (I could use a double sanitary tee but it's not ideal because it would mean dropping the softener p-trap to the floor level of the machine room, which is 18.5in higher than the garage floor). Many thanks.