Umm...I think you may be going a bit overboard with all the calculations for a little bit of residential duct work. The guy has what? Maybe 5 ft. of this duct to run? Six feet? He will have some pressure loss but jeez guys...it's not that big a deal. This probably doesn't need to be over analyzed.
Juss get'er dunn, times'a'wastin.
Uh-oh... I just realized that my above rationale might not make sense: whereas ONE 28x4 duct would have 64 (28+28+4+4) inches of perimeter, TWO 14x4 ducts would have 72 (14+14+4+4+14+14+4+4) inches! What the hell was I thinking? Well, it WOULD keep you far from the 7:1 max. length/width... but still you'd have the cross section surface area increase. I don't have a ductulator at home... let me look at this at work on Monday and get back with you.