one way to accomplish your goal would be to create a child theme (https://codex.wordpress.org/Child_Themes). You create a folder inside
/wp-content/themes/, then add a
style.css file inside it with a few comment lines at the top to tell WordPress this is a child theme of whatever your current theme is.
style.css file: all you need to do is change
parent-theme-folder-name to whatever your original theme folder is. So for example if you are currently using Twenty Sixteen, change
parent-theme-folder-name below to
Theme Name: Custom category sidebars
Next, to create your new sidebars, create a blank
functions.php inside your child theme folder, and add:
* Add custom sidebars for categories
// Agenda Category Sidebar
'name' => 'Agenda sidebar',
'id' => 'agenda-sidebar',
'description' => 'Agenda category sidebar'
// Articulos Category Sidebar
'name' => 'Articulos sidebar',
'id' => 'articulos-sidebar',
'description' => 'Articulos category sidebar'
Finally, create a blank
category-agenda.php file and a blank
category-articulos.php file. Copy and paste your original theme's
category.php code right into those files. Then, look for the sidebar portion, and change the part where it calls the sidebar:
Again in this example just change
sidebar-slug to your new slug: in
category-agenda.php change it to
agenda-sidebar, and in
category-articulos.php change it to
Once all these files are created and uploaded, activate your child theme and set up the widgets in the sidebars however you like.