Basically mirror out from the middle of the strip
Do you want it to exactly mirror? Would it be acceptable if the two sides were exactly mirrored (for example if both sides cannot be seen together from any angle)?
If so, here is a simple solution. Cut the strip in half. Reverse one half so that the data inputs of both strips are next to each other. Connect the same data signal to both halves. They will automatically mirror each other. In the code, set NUM_LEDS to half is current value.
PS: +1 Karma for using code tags. Now learn how to post links correctly also, so they can be clicked.