Adding Article to Multiple Categories
Open an article in the editor, and then open the article's settings by clicking the gear in the top-right.
Scroll down the settings until you see "Categories". Open the drop-down and select a category that you'd like the article to be added to.
Repeat this process to add an additional category. You'll know that you've done this correctly if you see both categories side by side (as pictured below)
Once satisfied, make sure to save and publish your changes!