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I'm not sure I understand your question. CPO is an extension of the factory warranty. If you have an issue that is to be covered by the warranty, any Porsche dealership will honor it. In the US, anyone can perform maintenance services on your car regardless of whether it's used, CPO warrantied, or new car warrantied.
I was thinking more of using a NON Porsche dealership. Or would that void the CPO?
For what? Regular maintenmaintenance? Performing services yourself or having an independent shop perform services does NOT void original or CPO warranties in the US so long as it was performed properly with parts that meet or exceeds manufacturer's standards. Just keep all your receipts. We have a law called the Magnusson Moss Warranty Act that protects consumers.
If you have a component failure that needs to be addressed and can be covered by the CPO warranty, then only a Porsche dealership can honor the CPO warranty and perform that repair.