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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.
Dougal
I posted a similar question about wastegate control on my compound 4JB1T a while back, I took your suggestion on controlling the HP turbo using full manifold pressure, and gating the atmospheric on its own
Have to say it was much better, boost came in earlier than the previous method of controlling to maintain a fixed P/R across the HP turbo.
I’m shortly setting up a Banks gauge with full data logging of temp and pressure at every stage to really get some firm data on turbo and intercooling and efficiency.
Paul
 

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I’m
Hate to go off topic, but how did the hopped up 4HE1 turn out? Considering that as an option as well, with an ASTR style starter.
Never proceeded any further with it unfortunately. Had some major issues I found with the particular engine I had, so returned it and got my money back. Sticking with the 2.8 4JB1T it’s still holding together, as well as having lot of spares. Bigger inline injection pump going on it soon. I have a 4BD1T sitting around now if I want to go bigger
 
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