Can co-executors of an estate whose assets are Under Trust by mutual agreement break the trust and distribute the assets prior to either of them dying?

The trustees must follow the provisions of the trust. If the trust doesn't provide that option or an option for the trustees to act at their discretion then the trustees must petition the court to modify the trust. They should be prepared to explain to the court why the trust should be modified. The judge will review the matter and issue a decision.