熊掌和鱼不可兼得,二者只能选其一"用英文怎么说

You can’t have your cake and eat it too.

这与您说的有异曲同工之处，也是英语中的习语。

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http://www.google.com/search?q=You%20can%E2%80%99t%20have%20your%20cake%20and%20eat%20it%20too&hl=zh-CN&newwindow=1&lr=&nxpt=20.284274850507739525524

如果要找英语中与其对等的谚语的话：
You cannot sell the cow and drink the milk.

英语谚语：

You cannot eat your cake and have it.鱼与熊掌，不可得兼。

You can't have your cake and eat it too

You can't have your cake and eat it too.
鱼与熊掌不可兼得。