Each project and situation is different, so it would be dangerous to generalize. First of all, ‘redundancy’ is a widely accepted engineering concept. Using two elements or components instead of just one increases what is called ‘failure tolerance’ i.e. the ability of the system being designed to recover from a malfunction. Same could be said about people – do you really want your commercial airliner to be operated by only one pilot? Or surgery performed by only one doctor? Some think that language interpretation is ‘just words.’ But in critical situations every word matters.
They understand it very well in diplomatic circles where 3 and not even 2 interpreters may be working as a team for each language. Think about the sheer number of words an interpreter works with on any given work day: 150 words per minute is the average speaking rate in the United States for English. Multiply that by 30 minutes (this is how long a simultaneous interpreter’s shift lasts) and you will get 4,500 words or 18 standard pages in just 30 minutes that you have to understand and reproduce flawlessly in a foreign language in real time on any topic. The cognitive load is immense! However, interpreters do not just sit passively after their shift: the interpreter listens to his working colleague, jots down new words and expressions, writes notes to his working colleague if needed, checks that terminology is accurate etc. Interpretation is very much teamwork!