Removing category meta box & adding 2 sub-category meta boxes

Hi,

I’m trying to split the Categories meta box on the write post page into two meta boxes (showing each all sub-categories of a specific category).

I tryed to add new meta boxes:

add_meta_box( 'my_sectionid', __( 'Categories', 'my_textdomain' ), 'cat_box1_inner_custom_box', 'post', 'side' );

and:

function cat_box1_inner_custom_box() {
	global $post, $categories;

  // Use nonce for verification

  echo '<input type="hidden" name="myplugin_noncename" id="myplugin_noncename" value="' .
    wp_create_nonce( plugin_basename(__FILE__) ) . '" />';

  // The actual fields for data entry
		$cats = wp_get_post_categories($post->ID); 

?>
	<ul id="category-tabs">
		<li class="tabs"><a href="#categories-events" tabindex="3"><?php _e( 'Event Categories' ); ?></a></li>
		<li class="hide-if-no-js"><a href="#categories-loc" tabindex="3"><?php _e( 'Locations' ); ?></a></li>
	</ul>

	<div id="categories-loc" class="tabs-panel" style="display: none;">
		<ul id="categorychecklist-loc" class="categorychecklist form-no-clear" >
	<?php 		wp_category_checklist($post->ID, 39, $cats); ?>
		</ul>
	</div>

	<div id="categories-events" class="tabs-panel">
		<ul id="categorychecklist-events" class="list:category categorychecklist form-no-clear">
	<?php
		$cats = wp_get_post_categories($post->ID); 

		wp_category_checklist($post->ID, 3, $cats); ?>
		</ul>
	</div>
<?php
} ?>

How do I save the selected categories? Someone able to help?
Thank you!

(I’m aware that this code / CSS IDs gets “in the way” of the normal category meta box.)