what is a linking literature in a theoretical framework? the linking of the 2 variables in a study/title or the 2 theories that is used in a study

so we were tasked to create a research title and the instructions were if you are going to use 2 theories, there must be a linking literature. Does it mean I just need the variables in my research title to have an existing study or do I need a study that has the 2 theories?