The reason for using a Deltagate in the crossover had more to do with the fact that one Deltagate at the turbo would not control boost properly.
That is why there were those who put one in the crossover as well. It was more of a boost control issue than it was anything else.
I have a dual set up on my car and it works just fine.
I am using a 60-1 turbo and it is only at that point that we ran into surge problems using the 60.
The second wastegate is mounted on the drivers side and I ran a dump pipe to dwonpipe right before it goes in to the catalytic converter.
Now you can one big wastegate, mount it at the header and it will control boost all you want. '
New tech, old tech.