I have only skimmed through the 14 pages of responses but did not see any mention of cold water diving where free-flows can be quite common. Having two people breathe off of one first stage can result in the donors reg free-flowing as well especially when the receiving diver is a little panicked and breathing heavily. I have seen a number of double free-flows and all too often they result in a rapid ascent. By cold water I mean sub 40 degrees.