IB says that a World Studies Extended Essay "invites students to conduct an in-depth, interdisciplinary investigation into an issue of contemporary global importance." Let's break that down...

In-depth:

Interdisciplinary:

Global Importance:

In the case of the World Studies essay, it means that you are using TWO subjects to investigate ONE issue. You are not conducting two subsequent investigations, each using a different DP subject; you are using the knowledge, concepts, and methods from two DP disciplines to investigate ONE issue. The two subjects are used together.

IB requires that you must be studying at least one of the subjects that you are using (and MDID concurs, and goes further: we want you to be studying both subjects, except in rare circumstances.)