Furnace intake on both floors
Actually it is quite common to have multiple air return ducts, with both heating and air conditioning it can save 20-30% by making slight adjustments and changing the flows. I usually suggest dampers on both the supply and return trunk lines for each level for the maximum savings, summer with ac more flow to the upper floor(s) in the winter more flow to the lower floors when heating. This balancing if done well with a air handler appropriately sized can save up to 30% so it may be uncommon in your area but higher end systems do have multiple intakes.