I think there's something lost in translation. The first stage regulator is attached to your tank valve, if air comes out of this device, you've reached the turn point in your dive. Losing gas through a HP port (i.e. SPG) is certainly inconvenient, but it's not the one I'm too concerned about. You should be concerned about a LP leak, as the LP leak is going to drain the gas much quicker. From studying the very hard to understand question, I've deduced the following:
I think what you really meant to ask is, "Should I put an inline shutoff on my 2nd stage regulators so that I don't have to close valves in the event of a free flowing regulator?" It is possible you might have seen someone do this who is a super-advanced awesomey diver. In fact, I might have been spotted doing it once or twice...but not with doubles. The only real [accepted] reason to rig your hoses this way is someone diving a rebreather who is using only onboard diluent with a regulator attached to the diluent bottle. You'd do this to preserve diluient which is doubling as bailout gas to an OC regulator on relative shallow dives, say 100ft or less maybe.
In your case there's absolutely no reason to add the inline shutoff valves because doubles already give you a satisfactory and positive way to preserve gas, and get to the entire volume of gas assuming you haven't extruded an o-ring at the tank neck (rare).
Try getting with a qualified technical instructor on this topic. I'm not sure you understand the exact failure modes/responses for various conditions in doubles. Some might even argue the pony bottle and the inline shutoff valves gives you too many avialable options and therefore Hick's Law (too many possibilies) might prevent your success. The only reason I can see for carrying a pony bottle with doubles is lack of understanding of your primary diving equipment. Say you grab your secondary and there's no gas, which problem do you need to solve the inline shutoff you can't see or the valve you can't see? The cave guys (much as myself) will quickly point out you've also managed to create another joint which is another failure point.
The transition to doubles does take a bit of time. Don't be fooled into thinking it's all about buoyancy. By the time you get to doubles, you should know how to compensate for a bunch of weight on your back. The proficient diver in doubles is fully self sufficient, and can assist a teammate without hestiation.
If you need deco/travel/stage gas, that's obviously a different issue.
