Instead of adding a sidebar, why not add the relevant HTML to the template itself? You can add the desired HTML to the template-testimonials.php file. Setting up sidebar on this page can really spoil the design since you have so much text in each of these testimonials, at least that is my opinion. Pls let me know if you see an issue with putting the HTML in the testimonials – possibly at the bottom of this page.
Ok I understand it better now. You will need to make two small code changes –
1) In template-testimonials.php file, add
at line number 142 – just before the last line.
2) In framework/presentation/layout-manager.php file, you should be able to locate template-testimonials.php file reference at line number 288. Remove the same so that the page becomes two column.
If you do not want to make these code changes in the parent theme and afraid of changes being lost, you can create a new template with a different php file name in the child theme and copy over all of the code from template-testimonials.php file and then add get_sidebar() to it. Should work well.