How to Use Block Linking
This is an experimental feature that isn't final yet and this might not be the final form of the feature, and it might still change in the future, or might get removed.
You have the option to link or unlink columns for convenience. If you link the columns, styling one column (even its inner blocks) will apply the styles to the other columns as well. If you unlink the columns, you will be able to style them independently.
You can enable Block Linking in the Stackable Settings in your WordPress dashboard. Just navigate to the Editor Settings then toggle on the Block Linking option.
Once you have enabled it, you can go to any page, add a Stackable block with columns, and you will see the linking icon on the top right of each column.
Linking the columns means that when you design one column, it will also apply the design to the other column.