Years and years ago a HVAC tech told me that one of the best things to do for a HVAC system was to add a second air return. He stated, basically, that doubling the size of the HVAC filter area would make the system quieter and put less stress on the HVAC components.
That was years ago, so...
Q: In a standard new, stick built home what are the ramifications of adding a second air return, that doubles the filter area, to the HVAC system?
Generally this is easy enough to do for the average home owner and costs < $50 for a “20 X 20 Steel Return Air Filter Grille for 1" Filter” (see Amazon).
 Any reputable builder who’s won something like: Professional Builder magazine--Builder of the Year or National Association of Home Builders (NAHB)--National Housing Quality Award
Edit 2018-08-27 11:11, Clarifications in reply to comments and answers:
Since there are multiple possible HVAC build outs where a homeowner can achieve the same thing, I was trying to not use any one specific build out. As it seems my question wasn't clear enough, I'll add what the 'goal' is (which I should have done originally) and one specific build out for reference. Hopefully that'll make a better understanding? I'm going to also add a second, related question at the end of this edit.
The HVAC change is to allow the homeowner to increase (double) the size of the return air filter area for less than $100.
Alternate Q wording:
Is doubling the size of the return air filter area, with no other changes, good or bad for a HVAC system?
A specific Example:
The existing HVAC return air filter is in standard(ish) drywall 'feed' ducting to the HVAC blower unit. In many new homes this return is located in a central hallway, or other central area, of the house and there is plenty of 'blank' drywall in this drywall 'feed' ducting to install a second 'Return Air Filter Grille for 1" Filter' assembly .
Worst case, to see exactly what the drywall 'feed' ducting looks like, you'll need to goto a stick built model home, and stick your head through where the HVAC filter is. Think of a rectangular return duct built with drywall...
Two other clarifications:
- The existing air filter is in a wall, and there is no air filter at/on the HVAC system itself.
- The new return is added to the existing return 'duct work,' so the only true change to the HVAC system is there is now twice the filter area.
 I don't know what the entirety of this component is called, it's the steel frame that holds the filter and has an airtight fitting into a wall. A unit that holds a 20x20x1 filter is less than $50 at Home Depot, Lowe's, Amazon, etc.
Second, related Q (feel free to expand as desired! ..might need a physicist, as this is definitely outside of my math skills..):
If the existing filter area is 20x20 (400 sq in.), and the builder recommends a MERV 8, now that the filter area is 2 times 20x20 (800 sq in.), can filters of MERV 11 or 13 be used without restricting the airflow below the original airflow?