ok so i want to make a box with 1 15 in the middle and 2 poorts, 1 on each side, to get it how i wanted it ported, could i just measure out the space i would need for 1 port and just put half of that on each side?

I believe you can't do it that way. You can figure out how big of a port you need by using this calculator http://www.carstereo.com/help/Articles.cfm?id=31

For # of ports you would want to put in 2, once you fill everything else out it will give you a length you need, and that is for each port.

Can't do it that way, instead design the box for having two ports.