transfer two existing taxonomy terms into one custom metabox

function my_meta_box(){
    add_meta_box( 'mymtx_div', 'This my own Meta box Only :-)', 'post_categories_meta_box', 'post', 'normal', 'high', array( 'taxonomy' => array(0=>'moviestax', 1=> 'bookstax') ));
}
add_action( 'admin_init', 'my_meta_box', 0 );
function post_categories_meta_box(){}

This example migth help you.

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