"Each other" and "one another" can be interchanged. They are virtually the same. The only small difference is that "each other" would be used if you were talking about two people. You would use "one another" when talking about more than two. "They patted each other on the back for a job well done." "In an emergency such as a tornado, neighbors should help one another." [Note that you could have switched the two phrases in the above sentences and there would have been no error.]