This is a sample, all lines have been trimmed leading and trailing spaces.
word word
word word word
word word word word
word
Result would be
word word
If you want to find lines that just have one space (with some 'word' before and after), try with this:
^\S+ \S+$
Explained:
^ # Begin of line
\S+ # non-blank character repeated 1 or more times
[ ] # a space (I'm using [ ] to make it more clear)
\S+ # non-blank character repeated 1 or more times
$ # end of line