Use a shortcode include plugin and simply add a shortcode inside a text widget in the sidebar you’ll be using on the widgets or customizer page.
Here are some example plugins and how they work:
Include Me –
[includeme file=”filename.php”]
Custom Content Shortcode – [load file=template/sidebar.html]
Insert Pages Plugin – [insert page="{slug}|{id}" display='title|link|content|all|{custom-template.php}']
Include Shortcode – [include slug="hello-world"]
You could also simply duplicate the template and rename it to sidebar-{pagename}
Duplicate page template, remove commented header inside the file, and rename it sidebar-custom.php.
Then just include that inside the other page template where you want to use it.
<?php get_sidebar ('custom'); ?>
Other methods if you don’t want to rename add another file just include it in your template like a sidebar:
(will return template-name.php)
include ('template-name.php');
(will return template-name.php)
include(get_query_template('template-name'));
(will return template-name.php)
locate_template(template-name.php, true);
(will return content-page.php)
get_template_part( 'content', 'page' );