If you know the basics of Google’s webmaster guidelines, you know that Google likes to see natural links–that is, links that are naturally earned, that aren’t meant to manipulate rankings.
Google knows that webmasters and businesses have a revenue model that includes linking to other sites that they have a commercial interest in, as in the case of affiliate marketing links and sponsored links (such as banner ads).
Enter the Link Spam Update
Google likes to see transparency, and the link spam update basically said that if you have a commercial interest in linking to another site, the link code should have a rel=sponsored attribute. (Or, the consensus is that a rel=nofollow is also good.)
But What If You Own 2 Sites That Link to Each Other?
But…what if you own two sites, of which you have a commercial interest in (since they’re part of your business) and you link from one to the other?
A participant explained his situation and asked a question: “About the link spam update: so we have a medical site which links to another site to allow the customer to book the doctors, which is on a separate domain. How would you treat these links under this link spam update–would you mark them as sponsored or would or you know we own both domains?”
John Mueller replied, “I would just use normal links. I think that’s like a completely natural link between those two sites. That’s kind of the functionality, if you will, part of the website. I will just use normal links for that.”
For clarification, the participant asked further: “So, it wouldn’t be seen as like an affiliate link by google because it’s going from one domain to another with a UTM or something like that?”
“No, no, that’s perfectly fine,” John replied.
So, if you own 2 sites and link from one to the other, even if you have a commercial interest, it’s basically fine. I would say that it’s fine because both sites are owned by the same party.