Futari is a native Japanese word meaning 'two people'. In Chinese characters it is written (which can also be read ninin in some circumstances).
This is a case where an etymologically unrelated Chinese compound has been brought in to write a native Japanese word. The reason is fairly simple: there are no Chinese characters that are suitable for writing the word futari. Consequently, the characters (or even more commonly when written across the page,) are applied to futari because they have the same meaning. This is an example of futari written :
Notice how this advertisement uses Chinese characters at every turn (kotoba, de-ai, suteki), even where many people would be happy to use hiragana. The exception is the final phrase, where itadakimasu is in hiragana rather than characters.
Other signs prefer to use hiragana for futari (). For instance:
A Google search during December 2003 found the following distribution on the Internet: