I read both this post and your other post.
I went through something very similar to this with a dive buddy, because of the way he layed on the inflator button to slow his descent while breathing -- as opposed to several short bursts between breaths -- his first stage froze up and started to freeflow. I agree with CMAN, in that instance no shutoff switch is going to solve the problem because both regulators were massively dumping air (and I mean just an incredible amount of bubbles). Shutting off one second stage in that case was only going to exacerbate the problem on the other second stage, and maybe the inflator as well. The underwater solution was to have him breathe off my primary and shut off his valve for a few minutes (though it seemed like hours) until the ambient water temps gradually thawed the first stage.
We both learned a lot that day including (in order): [1] use better cold water technique, [2] only use sealed diaphragm first stages in cold water, and [3] use doubles instead of single tanks, or at least two first stages per diver for cold water dives.
Scary stuff! I've always been an advocate of diving as a unit with your dive buddy, but that experience convinced me beyond a shadow of a doubt.
Glad everything worked out for you guys.
