Hi Nadia,
Thank you for reaching out to us. You can do this with Unlimited Sidebars feature where you can create a custom sidebar and assign it to specific pages only. To create and assign a custom sidebar to specific pages, please follow https://theme.co/docs/work-with-unlimited-sidebars#how-to-add-a-sidebar
If you’re trying add another sidebar then please note that two sidebars layout is currently not supported, we have 2 columned layout (sidebar left, content right and content left, sidebar right). What you could do is having 1 sidebar globally, and use Cornerstone to create another two column layout so one will be your content column and other would be the sidebar.
Hope this helps!