You can use Custom widget area instead of the Main sidebar (Pages > edit page > 8theme Layout Options > Custom sidebar) and just add the same widgets as you have in the main sidebar to custom sidebar. So, why do you want to display both sidebars?
Anyway, if you are sure that you need to edit this code you can find in xstore/sidebar.php. Copy this file into child theme and do the necessary changes.
Site works as blog, so we have a lot of posts. To set sidebar option one by one per post is not so good idea. I have Child theme and have sidebar.php for customization. Can you provide me the piece of code to add? The wish is to “display Main Sidebar and Custom Sidebar both by deafult in main sidebar area”.
Unfortunately, we are unable to provide support for customizations under our Support Policy. If you need help with PHP customization contact WPKraken team.
As I understand you want to show some content from the custom sidebar for the posts only and don’t show it for the all the other pages that also displays the main sidebar, right? You may use CSS workaround to hide some widgets for all the pages and show them for the single post only.
For example, add all the necessary widgets into the Main sidebar. Then use custom code to hide widgets by IDs for all the pages (display: none;) and then show them for the posts only (.single-post class + id and display: block;).