Display post content within related post

I’ve created two custom post types: Clients and Testimonials. I’d like to display the testimonial from Client A on the post for Client A, but also include it in a page of “All Testimonials”

I’m new to WordPress and thought there might be some way to “relate” two custom post types, but I also might be making this a lot harder than it needs to be. I looked for a plugin that would display the testimonial in the sidebar (perhaps a custom field?) and found Widget Logic, but I don’t know how to pull the testimonial content into the widget.

Any help would be appreciated. I’ve searched for answers but I don’t think I know the right language to use because I’m not finding much and I think this is a fairly common scenario.