There is only a certain amount of airflow that the intake pipework can handle, and the velocity of that airflow is entirely dependant on the Turbo's ability to effectively utilise the airflow. Assuming there is an element of 'Ram-air' due to the forward motion of the car.
If you have a Y piece then at the point to two airflows join an area of turbulence will be present, this will effectively stagnate the flow. I am assuming that Forge have tested this.
The extra airflow will however have some element of cooling the whole shebang down.
The angle of incidence of the 2 airflows will have a tremendous effect on the amount of turbulence present. The forge is about 90 degrees between the 2 airflows, this is quite a difference in flow directions.
Interesting.
....As I understand it, the 70mm piping will only flow a finite volume of air regardless of the number of filters used. Higher flowing air filters, however, provide for a reduction in effort required to draw air through them due to a decrease in flow restriction. A larger surface area through which airflow is introduced, provides for the least possible restriction in flow.
Okay, so just before this less restricted airflow has arrived at the 'Y' junction it passes through a velocity stack - Vanes moulded into the delivery end of the filter's cannister. This also has a bearing on why the Twintake orientates its cone-shaped filters the opposite way to usual practice.
Without having very in-depth discussion with Forge to explain it further, the airflow sucked in at the 'Y' doesn't suffer from stagnating. I'm told by Forge that they have thoroughly tested this and also a version with just the one forward facing filter cannister (which gave less gains).
I'm sorry if I can't be more help in answering your questions at the moment. I may be able to post some answers another day though.