Between two stools
Fail, due to being unable to choose between two alternatives.
This is an old phrase. It is first cited in John Gower's Confessio Amantis, 1390:
"Bot it is seid - Betwen tuo Stoles lyth the fal..."
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.