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Thanks for posting here, elisabetta-farin.
In QuickBooks Online, we can add more than one email in the Cc field. We just need to separate them with a comma (,).
Also, it seems that you're a developer since you mentioned API. If I'm right, I suggest visiting our Intuit Developer Community. They will be able to further assist you with it.
I'll be here if you need anything else. Have a wonderful day!