It's not a dumb question. Dreamwidth isn't terribly intuitive.

Go here: https://www.dreamwidth.org/customize/options, click on modules and select multilevel for tags



Then just use a colon to seperate the main tag category from the more detailed tag. I'm thinking of reorganizing my tags, but at the moment you can go to my journal for an example of how things like "fandom: discworld" and "fandom: the 4400" display.