multiple port placement....

im currently in the process of planning to build a vented enclosure for some subs i got. im using an 13x23 (internal) front baffle...with an 11" cutout for the sub...
the prob is im planning to use to 2 3" ports (cause i already got lots of 3" pvc lying around).

ive read that ports should be at least a diameter away from walls but does that also apply to another port also? or could i have the 2 port closer to each other?
 
It really depends on what you want, if your driver is going to be front firing and you are wanting the punch in the chest for movie watching then port toward the front as well. Anything that the air waves hits or the cabinet touches is going to resonate sympathetically and become another speaker. Room size, speaker location, building materials of the space all comes into play. How low the sub will be played and room size make a big difference. A 20 Hz wave length is over 20 meters long. Too small a space and you are inside an acoustic suspension speaker. Everthing affects everything so if you can go to some specialty HiFi stores and listen and look at porting etc and comparing to your space might help you decide what will work best.