A bit of MATH may help you.
A 6" diameter supply has a capacity of approx 28 inches of air. If you have 2 of these 6" supply pipes then you have a total of approx 56 of supply air trying to go into this 500 sq ft area. To obtain the best heating or cooling result this area will need at least the equivilent (56 ) of RETURN air from this area. (you can only put IN as much air as you can take OUT.) Note: a 4" pipe has a capacity of approx 12 in of air. A 5" pipe has a capacity of approx 19" ..... You do the MATH.