It is not recommended to have duplicate content, in your case if you need/want to use a post twice for two different blogs I suggest simply modifying one of the blog posts. You keep the same topic and that just reword a far amount of it.
How Google penalizes you will vary on how they see the situations, if you start getting a lot of duplicate content Google will likely think that one of you is stealing from the other. Usually the one that will get penalized is the one which has it's content crawled last, as to Google usually the first time it see the content it sort of "Notes" that it belongs to that website. The site that Google believes to be duplicating the content is usually penalized and in some cases removed from their index.
However that information i read quite some time ago and Google may have changed how they treat duplicate content but I would say that it is something along those lines.
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