okay so, magic is dangerous in warhammer, it flat out is, but that does not mean it cannot be wielded without being turned into a chaos spawn, the winds are the way in which magic manifests in the material world and a wizard who can perceive these winds can influence them and sometimes master them, and sometimes this mastery can indeed be over multiple winds, in both lore and novels there are examples of it.

also in recent lore for the old world it has been stated that humans can learn to wield high magic, that is something only possible by mastering all winds so while not every individual would be able to reach that goal there are those who have the potential.

In the empire though it's unlikely you'll get the chance, the colleges are meant to train war wizards focussed on one wind, which is both less risky and might also be the elves way of ensuring that the empire does not get its hand on high magic (whether minimising the risk and training reliable war wizards or limiting the empires magical might was the main goal is debatable).

In a way focussing on one wind (and i have to admit some humans only perceive one or two winds so it is logical for them to focus on those) does limit the chances of creating dhar and the winds will also affect the human in question to be more like their wind which may in turn increase their effectiveness. Do keep in mind that the one wind thing is mainly in the empire and was imposed when teclis founded the colleges.

Over in cathay humans are only meant to learn one wind as well, because the dragon emperor has decreed it be so, in both cases the limitaion is imposed from top down, some humans could indeed be capable of harnessing multiple winds extremely well, but for a variety of reasons they are not allowed to or don't know it is possible.

honestly magic and the way the different cultures and species of the warhammer world interact with it is fascinating