hi,
after reading this i can only suggest they are using a T-piece in the line from the wastegate actuator to the solenoid,and this is where the second MBC valve joins the system. if you sit down and think about it this should work, but the problem on later volvo's is that if you set the MBC at too high a pressure the map sensor will dectect that the boost is too high and try to shut it down ...which it cant do because the MBC wont open (as its set too high) so then the car will go into limp home mode,the check light will come on and you will get a code relating to excessive boost pressure etc...
just my 2 cents worth...

steve.