the total combined output is 160mm so there is no airflow restriction.
Just FYI:
One 120mm inlet:
120=2r, r=60
A=𝜋60^2, A=11309mm^2
Two 80mm outlets:
80=2r, r=40
A=𝜋60^2, A=5026mm^2
5026x2=10052mm^2
A deficit of 1257mm^2, so it is about 10% smaller.
For comparison, a 4" outlet (~100mm):
100=2r, r=50
A=𝜋50^2, A=7853mm^2
Yes changing the chamber to make them either slightly larger or one main chamber. Great info.