Yes the single 6" pipe has slightly more area than two 4" pipes
how much water shed area (roof and / or ground) are you draining with
I drain ~800 ft^2 of roof & another 800ft^2 of yard with a single 4"
line. The line drops about 20" over ~130 ft.
This setup handles almost all the various levels of rainfall I get in
SoCal. This year we had nearly 36" of total rainfall with some REALLY
heavy , in the 2" per hour range. With the soil at or close to
saturation, all this needed to be drained.
I did experience some flooding (temporary pooling) due to poor local
soil slope but the water would generally drain off in a few minutes
(10 min) after the downpour let up.
How water your pipe(s) can handle is a function of the size, type &
drop of the pipe.
IMO for most common residential applications a single 4" line should
work............but it depends on how much water your need to drain &
IMO simplest circular section area formula...............area = D * D
..7854 = Pi/4
because we tend to measure & think about diameters