Which system is more accurate for predicting relationship success?

Neither alone. MBTI reveals how you think and communicate; Enneagram reveals why you do what you do. Together they show communication style (MBTI) plus motivation and fear (Enneagram)—a much richer picture than either system alone.

Can I have the same MBTI and Enneagram type?

No, they measure different dimensions. You could be ENFJ Type 2, ENFJ Type 3, ENFJ Type 9—same personality preferences, different core motivations. This explains why two ENFJs can behave quite differently in relationships.

Should I prioritize one system over the other for dating?

Start with both. MBTI compatibility is useful for communication style alignment; Enneagram compatibility reveals if you'll support each other's growth or activate each other's fears. Best relationships have both cognitive alignment (MBTI) and emotional safety (Enneagram).