tandem

1. A carriage pulled by two or more draught animals (generally draught horses) harnessed one behind the other, both providing pulling power but only the animal in front being able to steer. (countable)
2. Two draught animals (generally draught horses) harnessed one behind the other. (countable, uncountable)
3. A thing with two components arranged one behind the other. (countable, uncountable) · If you want a canoe that can seat both of you comfortably, you’ll need a tandem.
4. A hollow metal tube containing radioactive material, inserted through the vagina into the uterus to treat gynecological cancer. (countable, uncountable)
5. A group of two or more machines, people, etc., working together; hence (uncountable), close collaboration. (countable, figuratively)
6. A method of language learning based on mutual exchange, where ideally each learner is a native speaker in the language the other person wants to learn. (figuratively, uncountable)

Plural tandems