¿Qué significa it takes two to tango ?

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Tango is a type of partner dance (2-person dance). "It takes two to Tango" means literally that "dancing the Tango requires two people." This expression is used to mean that "not just one of us/them is at fault."
Ex:
You: Did you hear that Joe broke up with Sally? He's such a bad person!
Me: Hey, it takes two to Tango (meaning: Hey, it was not just his fault, it was her fault too because she was equally involved in the relationship.)

Ex: brothers fighting
Bobby: Dad, Tommy hit me!
Dad: Why do you think he did that?
Bobby: I don't know! Tommy is mean!
Dad: Did you say something mean to him?
Bobby: Maybe...
Dad: Bobby, you know it takes two to Tango. You shouldn't provoke your brother like that. (meaning: well if you said something mean to your brother, it's also partly your fault that your brother hit you)