Another thing to consider is that the airflow that is moving through it is in pulses, same volume in as out with each stroke of the piston. Each down stroke moves the same volume as the up stroke. Top it off with the vacuum above the carb, it regulates how much oil is drawn out from the bottom nipple. If there was only one, then it would draw too much oil out and foul the plug. There is a constant vacuum being applied to the crankcase via those two nipples, and I believe one reason is to help draw fuel/oil up from the intake, through the valve guide, to help lubricate the rockers, this way there is lube coming from both ends. The exhaust rocker gets it's lube from the exhaust pressure. The lifters gets it's lube from the crank case, and some draw down from the rockers. Visualize this. The piston is on the upstroke, creating vacuum in the crank, which is already in a vacuum, the intake valve is closed, vacuum is drawing on the rocker side of the intake valve via the crankcase, but no vacuum is in the intake, so a bit of oil/fuel gets drawn up around the valve stem. Multiply that by 1000 when idling at 2000 RPM. The bottom nipple is open to atmosphere to help regulate on the downstroke so that it remains constant and eliminates any pulses in the intake.
One other thing to note, the Walbro style carb on a 4 stroke is not like that on a 2 stroke, it doesn't use crank case pulses to operate the fuel pump, instead it uses intake vacuum. They accomplished this by adding a spring to the diaphragm. When the intake is closed, vacuum drops enough for the spring to return the diaphragm.
On my other 4 strokes, Saito, I have the breather connected to a brass tube that is bent to follow the contour of the muffler with the end cut at 45 degrees right behind the exhaust opening. It acts as a scavenging system and helps pull waste oil out of the crankcase. Since it has only one opening, the pulses from the piston doesn't allow the oil to come out until the engine is stopped. By having the exhaust create a vacuum behind the 45 opening, it draws out the oil accumulating in the line, while still allowing air back into the tube with the upstrokes. The NGH is doing something similar by using the intake.