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IMO you're best to set the big turbo on it's own wastegate and the little turbo wastegate to the max boost you need.
This means the small turbo boosts higher initially and then scales down as the big comes on stream.

Your best efficiency comes from unloading the small turbo as much as possible. But this risks overheating the small turbos compressor wheel. So you've got to compromise by dropping the boost on the big one. I'd start with about 12psi on the big and see how you like it.

How the turbos interact is mostly about turbine size. You want the big turbine to be roughly twice the choke flow rating of the little. This lets the little spool up before the big starts to rob it's exhaust pressure difference.
 
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