<font color='#0000FF'>If you can't monitor an SPG then you shouldn't be diving would be my reply to anyone who got into that situation Mark. The twin has it's own redundancy if it is manifolded in that you can shutdown but obviously this doesn't cover running out of air. My pony on my single rig is for redundancy should I get a freeflow. DIR way is to have an H valve, makes sense as does most of it.
Opening statement may seem to be a bit harsh but that is my opinion and I am PADI.
