My thinking isn't that one jams and cannot be moved.
It is that one of them is acting unreliably and not returning to center. In the case of separate toggles this is noted immediately and can be felt by the pilot. With a single toggle the failure of one switch is masked by the ability of the other to return them both to center via the shared toggle. Rather than giving a warning that one of of the switches is on the road to failure, a single toggle masks that.