dual port box design for l7 15''?

If you use two ports they will occupy a greater amount of your enclosure than using one port.  Each port would be twice as long, using the same opening area, as one slotted port.  These subs need a large enclosure for vented, and you might not have the room to give up so much for the ports.

WinISD Pro has this driver in its database.  I looked at it and found that using just the one recommended slotted port will be okay (as far as port noise is concerned).

Look at the manual for this sub.  In the vented enclosure section you see three designs with different volumes.  I looked at the street design that uses 4.5 cu ft net airspace.  You have to measure your area and see how much room you have to work with.  If you have about 6 cu ft then this is a good one to use (you have to allow for the thickness of the MDF, the bracing, and the entire volume of the port incl. the port walls).  It appears that Kicker has figured for the driver displacement, but you would have to copy their cut sheet exactly as shown.  If your design's box volume differs from one of these examples, you have to figure for all displacements and add those to the cu ft you're targeting.

In that build, the slotted port is listed at 3" X 16.25" and with a length of 14.75".  That box will achieve high SPL levels with a tuning freq at 38 Hz.  Keep in mind this is pure SPL and won't be pleasant to listen to on an everyday level.

If you were to use two of these ports, each port would have to be 35" long.  But you could make each slot narrower by half if you need that symmetrical look.  To keep the same tuning freq you would make each slot 1.5" by the same height of 16.25", and increase each port length to 16.3".  Remember, you have to add total port volume to the enclosure, and two ports have more baffle walls that will count up in the total box size.

http://www.linearteam.dk/default.aspx?pageid=winisdpro

You can make the box shape anyway that fits as long as you attain the air volume you are targeting.  Remember that 12 X 12 X 12 = 1728;  that's the cu inches in a cu ft.