2 slides in on post

What is the url to where you are trying to do this? It should work without having to make any modifications.

I believe there is a problem because all the sliders have the same id (“#slides”), given in line 94 of class Slides_JS.
$slider_id = ($slide_type == 'gallery') ? 'products' : 'slides';

Just adding some unique string, like the slider-id ($id)
$slider_id = ($slide_type == 'gallery') ? 'products'.$id : 'slides'.$id;
solves this issue.

Of course this breaks the css, so adding a class “slides” on line 96
$html = '<div id="'.$slider_id.'" class="slides">';
and adding a lot of declarations in the css will be necessary.