Comment by dsacco

Right, that observation is correct on the surface. But the reason why that's almost never done is because the goal of a cryptographic algorithm is to contribute enough safety margin on its own. Instead of encrypting twice, it's better to encrypt with a greater number of rounds, or to come up with a superior algorithm. In practice you sacrifice an unreasonable amount of performance double encrypting in a production environment for a threat model that is fantastically unrealistic.