Fetching details from the dictionary...

combination; union; joining; incorporation; amalgamation; merger; fusion
combined; joint; united; shared
congruence
二つの会社が合同して新しい企業を設立した。
Two companies merged to establish a new corporation.
二つの会社が合同で事業を始めた。
The two companies started a joint business.
小学生と中学生の合同練習会が行われた。
A joint practice session for elementary and middle school students was held.