Looks like he is in sequential mode. At 28 seconds in the video, he is at 3500 RPM and floors it. right at about it 3800-4000 RPM the Turbo system seems to “transition” with the slight dip in boost. I believe while your in full boost with both twins online your only boosting to what ever that gauge goes too. But its not much More psi than while it was on the first turbo.
It almost like your in “stock” boost.
Here is a link while I was testing my 94 sequential system. Almost the same transition and it boosts to about 12 psi (the supra is stock)
Maybe you have a weak wastegate.