Miles you ask what seem like simple questions, but they really stretch the boundaries of explanation. Your question points to the source of misunderstanding and controversy about wide stance and two footed pressuring. Therein lies the answer to the real difference between PMTS and traditional skiing.
If you can ski in balance with forty percent pressure on the inside ski and sixty percent distribution on the stance ski, you should still be OK in many situations. I need to reinforce that there is a way to ski in balance with a fifty-one to forty-nine ratio between stance and inside foot, but I don?t recommend it. This is a precarious ratio with which to maintain balance. In this situation you are at the mercy of many forces and surface changes that are ready to disrupt balance in aggressive skiing.
If you become comfortable with this precarious distribution and make it status quo for your skiing, you will fall apart or ski tentatively on more challenging terrain, as support for outside ski edging from the core is limited with this distribution. Of course, the narrower your stance width the more likely you are to be able to maintain this very even weight distribution. As your feet get wider, you will end up losing your balance to the inside ski.
When you try carving with ratios near fifty-fifty, you are close to the limits of the leverage relationship for a level pelvis. When you ski with balance on the outside ski the pelvis is held on a more level plane. When the weight shifts to the inside ski lateral level hip and pelvis stabilization vis-?-vis the forces acting through the body, is no longer maintainable.
It doesn?t take much perturbation or disruption on the ski from the surface to carry energy back up the leg, into the hip, to unbalance the whole system. The pelvis has a critical function in maintaining body integrity, so it can act against the forces and stay angulated. The pelvis must be able to stay as level as possible. If it begins to tip toward the turn (lower on the inside and high on the outside of the body) the house of cards falls down. When pressure begins to shift to the inside foot, the forces no longer line up through the joints and limbs of the outside leg because the pressure is moving away from the edge of the outside ski to that of the inside ski. The pelvis begins to tilt with the outside hip lifting up, to align over the inside hip joint. The force line begins to move toward the outside of the inside hip. As you begin to support your weight on the inside leg, it tends to lift the pelvis, losing the favorable level pelvis and allowing the side of the pelvis over the outside foot to lift. The body has no resistance to this lift when in the compromised situation of balancing on the inside ski, the muscles that stabilize the tilting of the pelvis can no longer hold it level.
Miles, my response to your question therefore is that you are probably not really skiing with fifty-fifty distribution, but it may feel like you are. At a true fifty-fifty distribution you would find that outside ski balance is elusive. Remember perception is rarely reality in skiing.
In order to carry 50% pressure on the inside ski, one would have to keep the skis glued together and would have to use extreme counterbalancing and angulation of the upper body to maintain true balance on the outside ski, especially at the top of the arc.
This is particularly true if you are in powder with boot width skis (70 mm), with wider skis, say 88mm, you have more platform and forgiveness, but all the principles are still valid, except that you will find the ability to recover easier with 88mm skis or wider. The wide 88mm plus skis, don?t tip over as quickly, because the edges far to the sides of the boots have more resistance to tipping; it is most apparent in powder, we have more time and support to recover from balance errors. The wider skis in powder allow for rapid pressure development and can therefore be skied successfully with less ski and body angles.