Wikipedia
Multi-competence is a concept in second language acquisition formulated by Vivian Cook that refers to the knowledge of more than one language in one person's mind. From the multicompetence perspective, the different languages a person speaks are seen as one connected system, rather than each language being a separate system. People who speak a second language are seen as unique multilingual individuals, rather than people who have merely attached another language to their repertoire.
The concept has been backed up by studies showing how the different languages a person learns affect each other. The phenomenon of language transfer, or the first language affecting the second, has long been known. More recent research has also shown that the second language also affects the first in various subtle ways. There is also evidence that people who learn other languages gain general cognitive benefits.
Under multi-competence, the second language speaker is seen as more than the sum of the languages he speaks. This is in contrast with the assumption in much of second language research that the ideal model of a language is the monolingual native speaker. Setting the native speaker as the golden standard implies that second language speakers are somehow deficient in each language that they speak, whereas multi-competence sees them as having gained from learning a second language.
To avoid implying deficiency on the part of second language speakers, Cook prefers the term L2 user to L2 learner. An L2 user is anyone who knows a second language and uses it regularly, irrespective of their language level.