Mutually; with each other — an adverb indicating reciprocal action or relation.
彼らは互いに協力してプロジェクトを完成させた。
They cooperated with each other to complete the project.
意見を互いに交換し合った。
They exchanged opinions with one another.
互いに感謝の気持ちを伝えた。
They expressed their gratitude to each other.