How do I merge the two confluence sites into one?

We have two Confluence instances. Now we want to merge the two sites into one. Does this cause page or attachment conflicts? For example, two Confluences have a page with the same ID.
Or, how can we safely merge the two sites together?
Confluence(server)version is 6.6.0 and 6.4.1

You’ll probably want to ask this question over at the user community site:

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think you .