I really disagree here. If someone else is writing your unit tests, that means one of the following is true:

• the tests are written after the code is merged - there will be gaps, and the second dev will be lazy in writing those tests
• the tests are written before the code is worked on (TDD) - everything would take twice as long because each dev essentially needs to write the code again, and there's no way you're going to consistently cover everything the first time

Devs should write their tests, and reviewers should ensure the tests do a good job covering the logic. At the end of the day, the dev is responsible for the correctness of their code, so this makes the most sense to me.