Fake bidirectional MM table

Category.php

class Tx_ext_Domain_Model_Category extends Tx_Extbase_DomainObject_AbstractEntity {

	/**
	 * returns all the FAQs linked with this Category
	 *
	 * @return Tx_Extbase_Persistence_ObjectStorage<Tx_ext_Domain_Model_Faq>
	 */
	public function getFaqs() {
		return t3lib_div::makeInstance('Tx_ext_Domain_Repository_FaqRepository')->byCategory($this);
	}

}

FAQ.php

class Tx_ext_Domain_Model_Faq extends Tx_Extbase_DomainObject_AbstractEntity {

	/**
	 * Categories
	 *
	 * @var Tx_Extbase_Persistence_ObjectStorage<Tx_ext_Domain_Model_Category>
	 */
	protected $categories;

	/**
	 * Returns the categories
	 *
	 * @return Tx_Extbase_Persistence_ObjectStorage<Tx_ext_Domain_Model_Category> $categories
	 */
	public function getCategories() {
		return $this->categories;
	}

}

FaqRepository.php

class Tx_ext_Domain_Repository_FaqRepository extends Tx_Extbase_Persistence_Repository {

	public function byCategory(Tx_ext_Domain_Model_Category $category) {
		$query = $this->createQuery();
		$query->matching( $query->contains('categories', $category) );
		return $query->execute();
	}

}

template.html

your FAQ {faq} has the categories {faq.categories}

your Category {category} has the FAQs {category.faqs}