if you have a look at the code getStoreCategories calls a little down the line it uses Mage_Catalog_Model_Resource_Category_Tree
. Perhaps you can use it yourself like this:
<?php
require_once('app/Mage.php');
Mage::app();
$parent = Mage::app()->getStore()->getRootCategoryId();
$recursionLevel = 2;
$tree = Mage::getResourceModel('catalog/category_tree');
/* @var $tree Mage_Catalog_Model_Resource_Category_Tree */
$nodes = $tree->loadNode($parent)
->loadChildren($recursionLevel)
->getChildren();
$tree->addCollectionData(null, false, $parent);
foreach ($nodes as $node) {
/** @var $node Varien_Data_Tree_Node */
print_r($node->getData()); // first level category data
foreach ($node->getChildren() as $childNode) {
print_r($childNode->getData()); // second level category data
}
}