Alright. I think the outer dimensions of the box should 36"x36"x18". Make separate chambers for each subwoofer. Assuming that you will be using 3/4" MDF, the internal dimensions of each chamber should be 16.875"x16.875"x16.5". This will make each chamber 2.72 cu.ft.
Personally, I think you should use tube ports. PVC pipe works great. If you were to use 3.5" PVC for the ports, you'd need to make each port:
8" to be tuned at 30Hz
7.4" to be tuned at 31Hz
6.75" to be tuned at 32Hz
6.2" to be tuned at 33Hz
5.7" to be tuned at 34Hz
5.2" to be tuned at 35Hz
Well you get the idea...
If you want it to be tuned higher then that let me know. I'll get you the figures.
Hope this helps... //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif