A very useful tool is the "key" parameter for xtjs and it's also not mentioned
much in the documentation:

{xtjs_add key="uno"}
{xtjs_render key="uno"}

{xtjs_add key="due"}
{xtjs_render key="due"}

there's a problem in the class.jsloader.php file:

public function render(array $opts = null)
line 496: ... if( !$_hook_added ) { ...

it blocks the renderer from rendering the next item.
When commented out, i can use both xtjs_render as expected.