Trying to design a box for 2 12s. I want the port in the middle but Im not sure how that affects tuning.

Overall the box is 32w x 16h x 20d. Port is 3" wide. Sub displacement is about .3. Im aiming for mid 30s in tuning. I was looking to get about 24" of port length.

So the question is, when the port splits in two do you add both lengths on to the middle or do both sides act as one? ie: Its 16" down the middle then branches off 4" on each side. Is that 20" of port length or 24"?