I do not believe that this is a particularly good idea.
As a preliminary matter, I don't like pony bottles. The need for a redundant air supply is why doubles were invented and why I dive them.
As for this device, the only items that this product seems to eliminate from a traditional pony/stage configuration is the hose and the tank valve. I see no reason to do either. It seems that it will only make using the device harder.
For example, I'd rather have only the second stage in front of me. With this device, you'll have the tank as well.
Moreover, how do you hold it? Must you remove it, bring it in front of your face, then hold on to it while you ascend? That will be fun with a 30 cu. ft. bottle.
Finally, I see no way to turn this device off. Normally, I pressurize my stage regulator, then turn off the valve to that I won't lose deco gas while I'm on my back gas. It doesn't appear that this device allows this. The result could be an empty tank if, for example, the second stage leaks during the dive.
Overall, a bad idea.