just slide them both in and Velcro them down. no need to extend the packs leads, as that will add extra weight to your X5. make the Y harness on the ESC a bit longer on one side so you can reach the back pack. its not as far as you think. what i did was make my Y harness the same length, then made a 4" extension for one of the plugs to reach the back pack. i am not running all stick packs, so no need for that anymore.
fyi....i dont think battery companies even make a factory soldered up "12S" pack with a single lead anymore. most companies put both leads for the 6S packs out the front so it is just like charging (2) 6S packs.