I'm working on learning two languages right now! Great thread OP
I'm learning ancient Greek and French to prepare for my research degree, I'll also need to pick up Italian and Latin.
It's very interesting to learn a dead, ancient, literary language at the same time that you are learning a living, modern, spoken language. The funny thing about it is how much more extreme the ancient languages are compared to the modern languages
now bear in mind that I'm basically at the same stage of learning with both French and ancient Greek, and look at how different the content I'm learning is:
Ancient languages are so god damn serious
I'm just starting off and already I'm dealing with concepts like souls and gods, wars and peace, justice and evil. Meanwhile in French at the corresponding level I'm learning how to order coffee at a restaurant