Single Post (CPT) in two pages – Normal and Extended

The simplest way is done right in the post editor, you can use the button that is second in from the right side, of the editor toolbar, to insert a read more link at a certain point in your post. WordPress will display a shortened post with a read more link when the post is being viewed in a list of other posts in some category, for example. When you click on the more link, you are viewing that single post, and WordPress displays the full post content.

I’d definitely recommend reading this codex entry:

https://codex.wordpress.org/Customizing_the_Read_More#The_Excerpt_Basics

If this doesn’t work as described, please provide more code/info for debugging, or clarify further if this doesn’t meet your needs.