What are the word pairs that occured only twice?
word pair refers to w1st.word w2nd.word; occured only twice refers to occurrences = 2
SELECT T1.word, T3.word FROM words AS T1 INNER JOIN biwords AS T2 ON T1.wid = T2.w1st INNER JOIN words AS T3 ON T3.wid = T2.w2nd WHERE T2.occurrences = 2