I am looking for a reference whether the following situation is legit or not.

Suppose you have 2 website, A & B.
Each website promote a different product but on the same niche.
You create 2 different campaigns for the Google Content network only.
For all the Ads Groups you use the same website url using the placement feature.
For some of the Ads Groups you use unique themes of keywords and for some there is an overlap.

Will this consider as a double serving?

I would appreciate if you can support your opinion with Google reference.

In practice, I know people who do so for years and they never had a problem with that. However, I did not find any reference about this on Google website.