Say i had a box for a 12" and the port size was 3in and 5in long to hit about 60hz. Just if. Now what would happen if I just put two 3in ports that were 5 in long each? how would it affect it?
It is going to affect the sound drastically. A port is there to “tune” the sub to play in a certain frequency, and adding another port is going to throw that waaaay off. Unless you have a proper box-building computer program that you have entered the specs of your sub into, I highly advise you NOT to add the second port.
If you are trying to get a different frequency range, again, use a computer-based program to help you out, do not try and guess and check, you could actually push your subs excursion range way out, causing it to blow pretty quick.
HTH-