External wastegate to control high EMP on a sequential turbo setup

Hey guys I'm busy with a build on a N57d30B BMW 335D twin turbo diesel and have some boost issues and I'm open to good advice, will give some insight first.

Primary turbo is quite small spooling from around 1500 rpm with a VNT turbine housing.

Secondary turbo has been upgraded to a custom unit with a G35-1050 core and internal WG

So the way it works is on the stock cast manifold the primary is bolted on the top part of the manifold.

To the bottom is the Secondary turbo with a internal control flap cutting off flow from the manifold and primary turbo.

downpipe and intake is on the larger secondary turbo.

Problem is the transition between the two turbos are pretty big and causes a bit of a boost dip for about 700rpm.

On WOT the VNT closes immediately causing the primary turbo to spool, pretty quickly the EMP starts rising a lot as the flap is blocking off most of the flow.

so at about 3400RPM we open the control flap to dump all the EMP into the larger turbo which causes a spike in boost but unfortunately it does not have enough energy to overcome the inertia and keep the secondary turbo spooled so the boost plummets until 4000rpm and ramps back in and goes like hell through the gears with no issues.

the sensible thing is to keep the flap closed off a bit longer to get more flow to get the secondary turbo spooled immediate but the problem is it comes with insane amounts of EMP to the point where exhaust valve float is starting to stick its head out.

I have an idea and in theory it should work.

fitting an external WG on the manifold with the sole purpose of keeping EMP in a sweet spot so I can spool the smaller turbo without fighting emp and getting a more efficient spool up and transition.

that should make it possible to spool the Primary turbo longer to get more energy through to the secondary turbo in a better way.

current boost pressure is around 45psi but will go up to about 55 psi.

emp already hit 65+PSI currently.

reason I'm doing this setup is to have decent response for normal driving but more power up top.

Attached is a photo of a log referencing RPM/ boost pressure just for a bit of insight.

Something that has bothered me is how the boost drops in the transition, its quite immediate.

Any insight or opinions will be appreciated boys.