Understand shared port

[Rybass]

So i have searched high and low, and haven't found anything that tells me exactly what i need to know, so I'm here to ask lol.

If i am doing a enclosure with a 2 subs, port in the middle shared, how do I calculate length?

I'm going to have to make a t and run it along side the back of the enclosure, do I do each side to equal the length or half it?

Example....im doing a 3ft^3 Enclosure to 33 Hz. Program says with 125h and 3.5w, the port needs to be 30 inches long. So do i make each side equal

30, or to a do each t half of what it would be? And do i leave 3.5inch along back wall for each, or do I half that and do each t 1.75 inch away.

Sorry guys i cant find this info, and as simple as it is, i understand the complex shit, but couldn find answer on this. Thank u

[MrSkippyJ]

if the port needs to be 30 inches long, then each side needs to be 30 inches long. 

the only measurement that you change is the width. in your example the port would be 3.5 inches wide until it splits, then each side would be 1.75 inches wide. But each side would measure 30 inches from opening to opening.