Renovating 2 story home with separate HVAC systems, one upstairs and another downstairs. HVAC contractor wants to place the return airs high on the wall near the stairs going up and another in the ceiling for the 2nd. floor system above the stairs going down. The building will be very tight but the attic unit, including ductwork, will be in the unconditioned attic. Not sure if that makes any difference with regard to the RA. Attic will have foam at all ceiling penetrations and R38 cellulose. Location is New Orleans, so hot and humid 8 months out of 12. Thanks in advance for your reply.
Sounds like you are concerned about the placement of the 'return air vents' to these systems (one upstairs and one downstairs). Normally, if you have only a single Air Conditioner, the upstairs gets hotter than the downstairs (like my house! in Texas), and careful placement of the 'return vents' can help reduce this issue (put the upstairs return vent high on the wall, so it pulls in the warmest air).
Based on your description, I would not be concerned. Since you have a dedicated unit for upstairs, really you are golden. The fact that the downstairs return is 'near the stairs' means it is probably in a central location (which is perfect). You said the upstairs return would go 'in the ceiling' which is exactly where it should go since that will pull in the warmest air, and your air handler is in the attic anyway.