I noticed some traffic coming in from old links on a topic I wrote quite a long time ago about nesting Mootools Accordions. I am in no mood to resurrect a post from that long ago, especially since I know my way method was not optimal and I no long use or support Mootools. It’s all about jQuery baby! I will however, point you to an article that was inspired by my work, and greatly improved upon it.
You can read about the nesting of Mootools Accordions at Bogdan’s blog Medianotions. Remember, this article is a couple years old, so things may have changed, but it is a good read none-the-less.