It depends on the person's goals as to that why they want to learn a second language. If someone wants to learn a language for academic purposes, it would be better to master it. But if the main goal of learning a language is just to have fun and to speak with different nations, then a B1 for every language is enough. Even though people nowadays have the chance to learn different languages through the internet, if someone were to ask me, I would choose to learn one language at a time.