lanerd wrote:
When stuff is flushed, it should not allow any sufficient amount of air to come back up as the pressure it creates should cause the air to go up the vent...not up the toilet at the same time.  That's the whole purpose of the vent.  
Please try to explain to me how the TWO open pipes will not share any back pressure that exists; especially how the larger one won't take more during the time that it's open ????  HINT: The laws of physics are at work here.
The vent is there to prevent the BUILDUP of noxious gasses.
What happens when you open the big hole varies wildly depending on wind direction, windows open, etc.  Unless you have a power vent pipe with a fan in it......which I have never seen.