http://theconversation.i2p/what-makes-someone-bilingual-theres-no-easy-answer-162450
This is an idealised vision of a perfect, balanced bilingualism, assuming equivalent written and oral skills in both languages. According to this definition, a bilingual speaker is the sum of two monolinguals. However, this type of bilingualism is extremely rare, and in reality, bilingual people have varied language profiles.