Between two stools

This is an old proverbial phrase. The full version is ‘between two stools one falls to the ground’. It is first cited in John Gower’s Confessio Amantis, 1390:

“Thou farst [farest] as he betwen tuo stoles That wolde sitte and goth to grounde.”

The first recorded use in modern English is in Matthew Prior’s comic poem Alma; or, The Progress of the Mind, 1717:

Now which were wise, and which were fools?
Poor Alma sits between two stools;
The more she reads the more perplex’d,
The comment ruining the text:
Now fears, now hopes her doubtful fate.

See also: the List of Proverbs.