Amazon and Google do operate in several of the same markets. Amazon sells Kindle/Fire devices. Google sells Android phones. Amazon does product search. Google has a product search engine.
Antitrust enforcement focuses on the "primary market" of the entities. Google's primary market is advertising and Amazon's primary market is retail products. The DOJ does consider the potential anti-competitive impact on tertiary markets the two entities operate in but it wouldn't be a barrier to the hypothetical merger because those markets have plenty of other competitors.