Magento – make 2 parent categories and attach to them same sub category

Ok You shoud modify the function $this->getStoreCategories()

$_current_category = Mage::registry('current_category');
$path = $_current_category->getPath(); //get the path of where you are
$ids = explode('/', $path);
//now you have something like root category/base store category/first level 
if (isset($ids[2])) { //you should add some more complex lookout
    $topParent = Mage::getModel('catalog/category')->setStoreId(Mage::app()->getStore()->getId())->load($ids[2]);
//topParent is the ralph loren category even if you are in one of its children cat
}

Of course this is only a start point for your coding but you should have alla elements in it.

I recommend to no do this. You get in a lot of trouble, when you build a graph out of a tree, because you change the whole structure.

Instead just create a second category with the same name and put all the products into this.

Why do you want to do this?