1. Have you tried to apply a filter/segment with a whitespace character to include queries with 2+ words, and without a whitespace to include queries with a single word?
2. As an option, you can create a classification and apply a rule to classify the queries to one and 2+ words based on the whitespace inclusion. This will bring two additional dimensions that may be more convenient than (1) if this is going to be used on a regular basis.