Yes, you  need to include **both languages for each entry** in the _sitemap_: the url you are specifying in the `loc` item and every alternate language version of the page.

 Also the `hreflang="x-default"` is used only for auto-redirecting homepages or selectors, so in your case it seems that both pages have a specific language: `en` and `es`, it should be written with **hreflang="en"** instead of `x-default` like this:

    <url>
      <loc>https://www.example.com/en/about-us/testimonials</loc>
      <xhtml:link rel="alternate" hreflang="en" href="https://www.example.com/en/about-us/testimonials" />
      <xhtml:link rel="alternate" hreflang="es" href="https://www.example.com/es/about-us/testimonials" />
      <lastmod>2016-07-11T00:00:00Z</lastmod>
      <priority>0.8</priority>
    </url>
    <url>
      <loc>https://www.example.com/es/about-us/testimonials</loc>
      <xhtml:link rel="alternate" hreflang="en" href="https://www.example.com/en/about-us/testimonials" />
      <xhtml:link rel="alternate" hreflang="es" href="https://www.example.com/es/about-us/testimonials" />
      <lastmod>2016-07-11T00:00:00Z</lastmod>
      <priority>0.8</priority>
    </url>