"Always be of one mind and one mind" and "always be of one mind and one mind" are both evolved from ancient poems, and they are not idioms at the same time, so they are all right. These two words are regarded as a blessing in marriage life, expressing the harmonious relationship between husband and wife, mutual concern and family harmony; It is also often used to wish newlyweds happiness and longevity.
Source of ancient poetry:
"Nineteen Ancient Poems Picking Hibiscus on the River": "I am bent on leaving home and worried about death."
Li Deyu's "Yuanyang Pian" in Tang Dynasty: "Don't you see that people who have a heart in mind have become Yuanyang birds."
Qing Li in "Privatization of Smart Gathering": "How can there be more companions and less concentric?"
reference data
Ancient Poetry Network: http://www.gushiwen.org