Hi Angel,
You're still thinking in terms of diameters. The 16.25 sq. in. is AREA, not diameter. the 3.5" -3.75" is diameter. They represent approx 9.62 - 11.00 square inches...
If you need two exhaust openings, they should equal the same AREA as the single openings.
two 2.5" diameter openings would give about the same AREA as one 3.5" diameter opening.
(2.5x2.5)x.7854x2=9.81 square inches.......3.5x3.5x.7854=9.62 square inches)

As far as the idea of removing every other stator vane, I have also heard of people doing this, and I can't see why it wouldn't help. A Ramtec fan has many fewer stator vanes than a Dynamax has.
JA