Combining a VNT HP stage and wastegated LP can be a little tricky to get proper boost control with an OEM computer that takes only one signal (boost pressure) as input. You cannot control a vane position actuator and a wastegate actuator from same boost pressure signal, even if you were to convert so that both actuate by pressure or vacuum because they have very different control transfer function behaviours.
I hate to say this, but for most people, they should go with WG turbos on both stages to make their lives easier. Integral wastegates are fine, or you can use an integral WG on the HP turbo and external WG on the LP. You also cannot control two wastegates with a single actuation output signal, because stages need to be actuated separately and independently. Otherwise, both wastegates will actuate simultaneously, and both turbos will act as a single unit simultaneously combining the worst of the small (restrictive) and big (slow response) turbo's attributes. There's no one way to do it - it will involve some experimentation. Some people use boost pressure to actuate the HP (smaller) turbo's WG, while the LP (larger) is actuated by exhaust manifold pressure or interstage differential pressure.
As part of my Master thesis in 2009, I developed an R2S system for gasoline engines inspired from the similar system for Diesels in the BMW 23d with Borg Warner as collaborators:
https://www.gtisoft.com/wp-content/uploads/publication/FEV2009.pdf
Another alternative is to employ VNT on both HP and LP stages. This is a "latest" development published by Borg-Warner dated 2017, but I have documented done work in this forum on this dating back to at least
2009.
https://www.borgwarner.com/newsroom...rbocharging-system-with-two-vtg-turbochargers
In a "Bi-VNT" setup, the control can be the most challenging or most easy one of all, depending on your level of knowledge and comfort working with electronics and control systems. I detailed elsewhere where one could actually use a single N75 PWM signal from the OEM ECU to drive two separate electronic vane actuators. PWM signals are easy to manipulate and condition, so that you can modify the N75 signal to control the LP stage(e.g. change the frequency, introduce a lag or hysteresis, implement a switch, etc.), while keeping the original N75 signal intact to control the HP stage. Also, if you are an electronics wizard, you can add additional inputs to the ECU by adding a slave microcontroller (e.g. using a Raspberry Pi board) that takes additional engine RPM and throttle position sensor (TPS) signals to generate a map of when and how you want to the individual stages to operate. However, implementing this properly requires additional bypasses (e.g. external wastegate or butterfly valve) to deactivate the HP stage to prevent dangerous overboost.