Google was asked if having two websites and Google finding out about them would cause the websites to drop in rankings. John Mueller from Google said that is "not likely," adding that many people have several websites and that is fine.
This question came up in the April Google SEO office hours at the 32 second mark where Faith asked:
Question: My rankings have dropped due to Google finding out I have two websites. Is this correct?
Answer: No. That's not likely. Many people have several websites. Separate websites are not a problem. The issue is often more indirect. If you work on a lot of websites, you're not going to have a lot of time to make truly awesome websites everywhere. And, if you're making websites that aren't awesome, then that can be something which our algorithms pick up on when it comes to recommending your site to others.
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If you do spam, it is not recommended to put all your spammy sites in on Search Console account. It is a common question asking if Google will look at patterns between Search Console verified properties. We covered it back in 2009 and then when Matt Cutts talked about this topic in 2014. We also covered that many SEOs have access to many web sites, many with penalties and it may not be a concern but it is something you should look at.
Forum discussion at X.
