Still, I'm somewhat sympathetic to your question. I usually like to carry two kinds of fluids when hiking or bikingwater, then something with calories such as a sports drink or (better yet) my favorite, pineapple-orange-banana juice. Boy, is that great stuff. Wonderful flavor, calories, carbs from the banana, electrolytes. Try it some time.
So what to do? An enterprising mechanical engineer sort could figure this out: Take two hydration bladders, join the outlet tubes at a convenient point, install a two-way valve of some sort, run that into a single drinking tube, and you've got your two-liquid system. I should think a trip to a laboratory-supply store would yield what you need.
Barring that, there's no reason not to slip an extra bladder in your hydration pack and simply hang another supply line. I mean, why not? The extra weight would be minimal, at least for the apparatus. By the time you're carrying an extra liter or two of water, though... that might be a bit of a load.
Anyway, get to work. And when the Scott's Duo-Drink Bladder System hits the market, I want a piece of the action.
For expert reviews of on-the-market hydro gear, check out Outside Online's Hydration Systems Buying Guide.
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