All the responses here seem to rely on the math, but the concept here is really simple even if you don't know the equations that govern this system. In fact, the equations come from simple intuitive intuitive concepts, rather than the other way around.
To explain, if you have two skiers the same mass on the same skis skiing next to each other, of course they go the same speed down the slope. Then imagine they hold hands. You can now think of them as one skier with twice the mass — and they're going the same speed.
Now you might complain that they're on four skis instead of two. So imagine just one skier again, and imagine she alternates between skiing on both skis and skiing on only one. Now, this doesn't work perfectly (because the pressure actually changes the coefficient of friction by changing the heat transfer and the way the skis alter the crystals in the snow), but the basic idea is that there's *half* the surface area to slow the skier down, but there's *twice* the normal force. So those cancel out.
So put it together, and have two skiers, each on one ski, holding hands, and their speed is the same as for one skier twice the weight on two skis, or each individual skier on two skis, or each individual skier on one ski.
tl;dr: Imagine two skiers holding hands.
