I saw mention of a snorkel that has two separate tubes, one for intake and one for exhaust. I thought it was probably a gimmick, but decided to look for more info. Here's a good discussion about it on deeperblue Kapitol Reef Snorkel If anyone has more info about it, perhaps a double tube snorkel from a different manufacturer, feel free to add to this thread.
For information on what makes a good snorkel see this thread What makes a good spearfishing freediving snorkel