Short answer: Avoid it.
You want to avoid duplicating any kind of copy from one website to another as much as possible. If there's a need to do it (for instance, a blog post from the parent company could be useful to the audience of a smaller company) there's always the ability to use a canonical to identify the preferred (original) source in order to help Google identify which one should be given credit/indexed. Even with that, a canonical is simply a suggestion and not a definite order. Google can choose to ignore it if it wishes.
But in the instance of website copy that talks about a product or service where you want the ability for both pages to rank, I would invest the time in rewriting the content so it's specific to that website and avoid duplication as much as possible. As an example, the company that provides after-sales support has a definite point of differentiation. That could become a good chunk of the copy to help dilute it, along with rewriting the other parts of the copy as well. If you're trying to rank both websites for similar (or the same) keywords, you're going to end up forcing Google to make a choice between the two if a good chunk of the second website is just a replication of the first.
Investing the time in unique copy will be worth it. Hope this helps.