One way you can do this ethically is to join a goodreads review round:
https://www.goodreads.com/group/show/78683-review-group. Groups with "now forming" in the title are open for joining. Basically, you read four books and receive four reviews. In the group of ten reviewers, a moderator makes sure that no reciprocal reviews happen. Amazon is aware of goodreads reading rounds, and they don't violate their terms of service.
Much better than trying something that could annoy Amazon. Reciprocal reviews are often deleted anyway.