The antonyms for the phrase "more single-hearted" can be "double-hearted," "disloyal," "unfaithful," "false," and "duplicitous." These words describe individuals who are not committed to a cause, person, or belief. They may be deceitful and lack integrity in their actions, and their motives may be driven by personal gain or selfish interests. Being double-hearted can also mean being indecisive, uncertain, or confused about one's loyalties, leading to a lack of true devotion to a cause or person. Therefore, it is important to be aware of these antonyms and strive towards being single-hearted, sincere, and faithful in all aspects of our lives.