It depends.
To make a long story short(er), I feel that a two-handed grip is a crutch. Shooting may seem easier, but personally, I can shoot one-handed better than two under the right conditions. Weak hand support covers up a multitude of sins that you just can't commit with one hand, and gives you the perception of a steadier hold. But that crutch isn't going to fix something like improper grip and arm tension.
If I'm hobbled by a gun with a not-quite-match-ready trigger, then the crutch of my support hand covers up errors that would be induced by equipment.
With a clean trigger, though, the second hand is just a distraction.
PS--On arm and grip tension, I don't think that physical strength is relevant. If you have a weak person, and they squeeze with their hand, wrist, and forearm right to the brink of trembling, you're going to get good results. If you take a strong person, and they apply the same amount of force, which requires less effort because they're stronger, you get poor results, because their muscles aren't in the right state.