If you have ever undertaken the task of applying tape and drywall compound on an inside corner, you know it is hard to work on one side without bothering the other side. You may not realize it, but there exist special spatulas for this job. The 'inside corner spatula' has two faces that are spread out a bit more than 90 degrees so that when you apply a bit of pressure, they snugly smooth out the compound on both sides at the same time.
Another spatula does the same thing for the outside corners.