A right Scotchman has always two strings to his bow, and is in utrumque paratus
Tobias Smollett. The Expedition of Humphry Clinker (1771)
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"Well, you know that girl Sissie, she's a clever one, she is," he went on after a minute, staring at me. "She's a regular clinker! Got two strings to her bow; that's where the trouble comes in. Me and another fellow. She likes me for love and the other fellow for money. Now, don't you come and tell me that YOU are the other fellow."
Grant Allen. Hilda Wade: A Woman with Tenacity of Purpose (1899)
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my counsel is that thou have more strings to thy bow than one. It is safe riding at two anchors, a fire divided in twain burns slower, a fountain running into many rivers is of less force, the mind enamoured on two women is less affected with desire, [...].
John Lyly. Euphues: The Anatomy of Wit (1578)
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What is he foostering over that change for? Sees me looking. Eye out for other fellow always. Good fallback. Two strings to her bow.