Google does not have a penalty for duplicate content, but may decide to only fully list one version of the page.

This could end with severe damage to the pages Google chooses NOT to fully list.

Content is King; and Google takes that very seriously. Read it as "Unique Content is King".

It is best practice to develop each site searately, on its own merits; indeed, it is asking for trouble not to.

Especially in a highly competitive area, where every downside is a cliff edge.