It takes two to do something
Definition 1 of 2
Pro Tip 1/3Partnership Required
The phrase is often used when discussing activities that cannot be done alone. 
Starting a business can be tough; it takes two to truly get it off the ground.
Pro Tip 2/3Collaborative Spirit
Implies that cooperation and working together are essential for success. 
For peace negotiations to work, it takes two to seriously engage.
Pro Tip 3/3Mutual Effort
Highlights that both parties must contribute effort or resources. 
To raise a child well, it takes two to provide love and guidance.