Although I can't speak to learning two spoken languages simultaneously, learning a language similar to one I already knew (I knew Spanish, tried to learn Italian) was insanely difficult, because my brain couldn't distinguish them enough. It would get in a loop of searching for Italian words and running into Spanish words and then mixing them up.

On the other hand, learning German after already knowing Spanish was much easier -- and it was much easier to see the similarities and differences between the languages, because my brain would find the Spanish phrase while searching for the German one, and vice versa, but wouldn't get stuck in a loop about it.