If I understand you correctly, no, you can’t treat a post like a category and associate other posts with it.
It sounds like you created this second custom post type because you wanted to associate content with each ‘category post’.
If I understand your needs correctly, I think you’ll be better served by registering that second post type as a custom taxonomy instead. The registration process is similar as that for post types: if you’re doing it in code you can use the register_taxonomy function (see the Codex at https://codex.wordpress.org/Function_Reference/register_taxonomy for documentation) or there are many plugins that will let you do it as well.
As for associating content with your custom taxonomy, I think what you might be looking for here is a plugin like Advanced Custom Fields which, among other things, allows you to add custom fields to a taxonomy term – so you can still associate WYSIWYG content, images, text fields, etc etc.
Trust this solves the issue you have – but if it doesn’t, perhaps you could outline more what you are trying to achieve with the solution you proposed?