While traveling in Iceland I always wondered why most of the "monster trucks" had big tires with two valves.
So here my first guess why:
When you're wheeling on rocky roads, usually you lower air pression to have a better grip. The more you lower, the better traction you get if rocks are getting nasty. But at some point, you risk losing your tire from the rim. Without proper tools, it will be difficult to get that tire back on the rim because it's hard to inflate it. I know, there are some fancy tricks to get them back on, but in any case, it means that you must work for a couple of minutes.
To prevent this, some genius invented the inner air beadlock system. It’s a small, belt-like tube that fits between tire and rims. It always has enough pressure to prevent the tire from slipping on the rim. The rest of the tire-chamber has another, lower pressure that is meant for driving off-road. Once back on a paved road, all you must do, is pump up the tire to the desired pressure again. In Iceland, most of the folks carry an air compressor with themselves.
Second guess:
One of the valves is used to "fast"-deflate the tire while on the other valve you can check what pressure remains in the tire. There are also some valves I have read about, that you can use to "fast"-deflate a tire to a certain pressure without standing next to the wheel surveilling the process. It stops automatically when the pressure has reached.
What's your guess? Some Iceland folks that can reveal the mystery??
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