To best answer this question, I should probably also ask about the situation that inspired it. If a situation were to arise that it was considered favorable to "double up" then it's also reasonable that is the precise favor they wished to avoid.
No situation, really, except for doing some character building and thinking about ways to build more focused/unique characters. That, and I always felt the sorcerer (few powers, many uses)/wizard (many powers, few uses) split to be tactically interesting.