Even if I make a deliberate effort it's not especially easy to breathe out through my nose and mouth at the same time. I think there's a bit more resistance through the nose, so the air takes the easier path out the mouth unless I close my mouth a bit to create more resistance. With a mask creating resistance for air coming out the nose I'd think it would be very difficult to have air coming out the nose at the same time. Perhaps you're sticking your tongue in the regulator or something to restrict free airflow through the reg's exhaust.
As for having air come out the nose while inhaling through the mouth, I'd expect that to be physically impossible under normal circumstances. When you inhale the expanding volume of your chest cavity reduces the air pressure in your lungs, so atmospheric pressure
pushes air into your lungs. Short of a rapidly alternating change from inhaling to exhaling there's no pressure differential to push the air out your nose. Maybe a musician who's very good at circular breathing could push air out the nose while inhaling, but normally they'd be inhaling through the nose while using their cheeks to push air out the mouth.
Inhaling from a scuba reg is a bit different, since the pressure in the 2nd stage is far higher than any pressure you could generate in your lungs. When you start to inhale the reduced pressure opens the valve in the 2nd stage, so the air is then pushed out of the hose at about 130+ psi over ambient pressure. Perhaps you've discovered that if you purge a good reg the airflow doesn't stop on its own, but requires you to cover the mouthpiece to create some back pressure. It should only take a very small change in pressure to open or close the valve. That's why a good reg will free flow with the mouthpiece turned up in water, and (usually) stop when the mouthpiece is turned down - just the difference from a couple of inches of water is enough.
Theoretically, if the path through your nose is very unrestricted I suppose the reg could blow some air out your nose as you're inhaling. The problem is that the pressure in your lungs is lower and the mask means the path through the nose is restricted, so the air want to take the path of least resistance and just go to your lungs. Maybe when you stop inhaling the reg could flow for just a split second longer, and the air escapes through your nose since you've closed the path to the lungs.
I'm fairly sure this would require a mask that's fairly loose, and perhaps even fairly leaky, along with the path between your mouth and nose completely open. There's another recent thread that discusses being conscious of how to control the closure between the mouth and nose:
http://www.scubaboard.com/forums/ne...e-replace-technique-newbie-itis-revealed.html
Reading it certainly won't hurt any.
