Ramifications of adding a second HVAC air return (in parallel)?
It is true multiple points of return will increase the efficiency at a very low cost. Here is a common example from 1960's era tract homes. The single air return was normally in the hall way that lead to the bedrooms all rooms had heat registers. The kitchen and living rooms had large open areas that connected to the hall. When doing remodels we added a secondary return at the opposite end of the home usually 14x18" if memory serves, this helped to balance the draw through the home especially if the bed and bathroom doors were closed and reducing the draft and a minor reduction of noise at the return. Many homes only have 1 return, and many only have filters at the furnace. By installing filters at the original return and adding a second return with filters we reduced the pressure drop and kept the ducts clean compared to filtering at the furnace. We started doing this in the late 70's on remodels, it was not that expensive back then, even cheaper with flex ducting today. we started doing it to create a more modern look and eliminate drafts, the clean ducts became part of the resale pitch later. The largest win in my opinion is drawing multiple points keeps the smells in the kitchen...