Most people prefer not to use two bags per one comic. When you're talking about high-value comics it makes more sense, but even then, unless you test the bags and boards in question, there can be slippage, which I think would bother a lot of collectors. I've seen people post here about doing something very similar to what you suggest, though. I think it becomes a matter of thinking that if one bag is standard protection, two bags ought to be a higher standard for a higher value comic.
Also, there is a suggestion that the pH-balanced board actually helps deter yellowing, rather than simply not contributing to it, sort of like the micro-chamber paper.