cawzt3, I doubt you'll read this far, but nothing about your profile or prior posts suggests that this one was anything but a sincere question. The reason you have gotten the answers that you have is that it's a rather bizarre solution to propose to what I assume is the common problem of finding you aren't carrying enough gas to do what you want to do underwater.
Whenever you consider an equipment setup, it's a good idea to ask yourself what the problem is that you are trying to solve. If it's "more gas", then you can buy larger tanks, or double up tanks, or dive independent doubles, or work on methods of reducing your gas consumption (not a real option for spearfishermen, as I understand things). Then you have to look at the problems you CAUSE by the new or different equipment you introduce in your solution. For example, using manifolded doubles introduces a bunch of failure points, and requires that you learn to do valve shutdowns if the manifold is to be of any benefit to you.
In what you have proposed, you gain access to a second 100 cubic feet of gas without having to buy a new BC or an additional regulator. However, you DID have to buy a transfill whip (they aren't cheap) and two H-valves (which aren't cheap, either). You have put a second tank in a place where you're not going to find it very comfortable to carry it, and as I try to imagine this, I would think it would make gearing up very awkward, as well. And you're still going to have to learn how to do shutdowns behind your head, because any leak in the whip or its connections to the two valves will have to be managed by shutting down the second valve on your primary tank. So you didn't gain any ease of management there. And because the two tanks are connected by a long and flexible high pressure hose, the connections of the fill whip to the two H-valves are very vulnerable to anything that might catch and pull on the whip.
So, I think you have created a great deal of complexity, with its attendant potentials for failures and problems, and you have saved little or no money, except being able to use your existing BC. I suspect, if you sat down and penciled it out, that you could probably pick up a used HP130 for what it would cost you to put this all together, and you would keep a simple and streamlined system.
As far as your deco status with a pony bottle, the two are unrelated. Your air integrated computer monitors the gas in your primary tank. It also monitors your depth and time, and tells you how much no-deco time you have remaining according to the mathematical algorithm built into the computer. The two functions (gas and deco) are separate. A pony bottle is carried as emergency bailout gas, and not as a part of the gas plan of the dive. When you have to go to your pony, you are terminating the dive as expeditiously as feasible.
Since your computer will not tell you the gas in your pony bottle, you need some kind of gauge on it, so you can know it is full when you start the dive (and depending on how you carry it, to monitor that it continues to be full).
Please forgive all the sarcasm that's come your way. Creative thinking CAN sometimes invent a better mousetrap, but in diving, it often doesn't. If you're having a problem, chances are you aren't the first one to have had it, and somebody's probably tried a bunch of solutions to it before now. Asking the original question, "How do I increase my gas supply without having to buy a bunch of new gear?" will often get you some useful information without getting you beaten about the head
