Is it totally out of the question for two split sytems to share a return air? I'm looking at replacing two furnaces (3 ton & 90k BTU 80% AFUE maybe 95%) and two condensers (16 SEER). The furnaces are located to the left and the right of return air duct in the attic basically in the form of a "T". The openings for the return air are on the corner of the upstairs hallway and the stairwell. The opening on the stairwell is 20x25 and the opening in the hallway is 16x25. Both openings are for the same return air duct. The return air duct is about 20x20. If both furnances or condensors are running at the same time would the return air be large enough? How much would it impact efficiency (all units would be matched appropriately to get 16 SEER)? I've received various answers from different contractors. Thanks in advanced.