Both of these are idioms that are pretty common in English. However, I wouldn't say they were the same, nor have the same origins. "Thinking outside the box" is usually applied to a creative approach that no one else has attempted. It is often used when someone creates a new process or discovers a novel approach to solving a problem. This phrase means something is unconventional, unique, and entirely different. "Pushing the envelope" describes testing limits and boundaries. This phrase originated with aviation and the testing the performance of an aircraft or it's "flight envelope". Unlike "thinking outside the box", this phrase has to do more about finding, meeting, and exceeding the current limitations with conventional methods.