@lorenzoquintavalle @eff I was wondering the same. And after looking on their website, and in the settings of both extensions, I believe they both do the same thing in different ways.

The main difference being that PrivacyBadger easier to use (no block-lists to setup) and will only block ads that track you.

So you only need one extension.

Source: https://privacybadger.org/#How-is-Privacy-Badger-different-from-Disconnect%2c-Adblock-Plus%2c-Ghostery%2c-and-other-blocking-extensions