Here is an example:
I tell someone I want a dog. My wife tells someone she wants a dog. We have not agreed to get a dog.
In discussing together getting a dog, we cannot agree on who will walk the dog or who will train the dog. I say my wife and she says me. My wife then decides she doesn't want a dog after all.
No agreement was formed so no agreement was breached.
If you want a 50/50 split of whatever, I suggest you (with help from your attorney) make this 50/50 agreement an attractive one for the other party - perhaps by agreeing to a 40/60 split on something else.
In other words, negotiate. And when you come to terms that are satisfactory to both, get the agreement in writing and signed.