it takes two to tango

「相手もその気がなければ何も起こらない」。何か争いごとがあるときに片方だけが悪い訳ではない、というニュアンスなこともあれば、「どんなにがんばっても独り相撲じゃダメ」というニュアンスなこともあります。

Startup Hacks: An Early Stage Checklist from Mashable

Keep in mind that 80% of business development deals don’t yield the level of outcome you think they will. It takes two to tango, and often times you’re wrestling with an elephant that isn’t going to change on your account.

"An elephant" refers to a big corporation that could be a business partner. If that partnership happens, that's great. But it usually doesn't.