Again, the article has a specific definition of what it means to speak "one language" - it means to renounce all knowledge trapped inside the dead language, cul

Again, the article has a specific definition of what it means to speak "one language" - it means to renounce all knowledge trapped inside the dead language, cultural and all. Therefore yes, if you both only spoke English (in the absolute case at issue) you woudn't have different cultures, by definition. Language and culture are not linearly independent.