are you co-sleeping? i'm not sure how it would work with twins, but the nursing + co-sleeping mamas i know are able to half-sleep through the night nursing sessions at 6 months, since the babe can find the tap themselves at that age. like i said, i'm not sure if it would work for you, you've got two taps to run
i don't have a copy of that book, but i saw it an an LLL meeting. contact your local LLL and see if they have it in their lending library, actually they might have several different books on night-time parenting.
here is an article at Dr. Sears' website about night-weaning the "all-night nurser" it may apply to your situation. hope this helps! wishing you sweet dreams and good sleep