In today's dictionaries 二人三脚 is the race in which two people run side by side with their inner ankles tied. Its second sense is two people working together to see a thing through. The four characters say two people, three legs.
Lectures, advice books and wedding speeches have said 二人三脚 of a husband and wife since at least 1917. Bound like the pair in the race, the two go as fast as their step agrees, and if one falls, both fall.
The dictionaries' examples of 二人三脚 include a bank and a client firm. They also include the two priests of a temple, and a student and his teacher. Whether it is for a marriage or for a partnership of any kind is yours.
The three-legged race was on the program of the meeting usually counted as Japan's first athletic meeting, held under British naval instructors at the Imperial Naval College in Tokyo in March 1874, where the program named it with a line in the manner of Chinese verse, the butterfly chasing flowers; the word 二人三脚 is in print by 1885, and the earliest likening of husband and wife to a pair in the race in the Diet Library's full text, as searched for this history, is a lecturer's of 1917.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 二人三脚.