Hi, I currently am learning Italian, Dutch, Spanish and Swedish simultaneously. For me, it works very well but I guess it depends on what your goal is. For me the most important part are reading skills because I love reading works of literature in their original language. Watching undubbed foreign movies is the second most important skill for me. So, I mainly 'consume' the languages and don't write or speak a lot in them. In this setting I think it actually helps to learn several languages simultaneously because there are a lot of similarities between them and you can accommodate very quickly to new concepts or vocabulary via analogies in one of the other languages. If on the other hand your main goal in learning a language is to speak and write in it, I guess it probably is better to focus on just one at a time because then the aforementioned similarities can become very confusing.

interesting. I have almost no background with any language (other than English) so I know that trying to tackle two would be more work than one. I'm not sure it would be twice as much though. Much of the learning is not remembering the words but thinking about verb conjugations and pronoun use. Of course, when I speak English I never bother about these; as a native speaker I simply say what comes naturally - right or wrong. Now that I am forced to choose words with care, I'm not sure that learning two vocabularies would be that much more effort than one.

I am trying to keep my skills in Spanish, Russian, and Finnish while learn French (and maybe German). Those languages are different enough that it is easy to remember the difference, but as someone who excels in languages I think that knowing multiple languages actually improves learning others, even at the same time, because you can compare, contrast, and relate concepts and words between the languages. Of course, there are times when I think there should be a word that really doesn't exist in one language because it exists in another. As long as you keep studying and maintaining your skills you won't have a problem.

I started picking up Portugese after I was well on my way to being fluent in Spanish. It got me a little mixed up so I stopped. They are very similar languages though. I'd say if they are very different then it should just take that much more effort to really dedicate yourself to two languages.

I'm working on French, German and Spanish at the moment. These are going ok, but I took the equivalent of a year of French and the equivalent of a year of German reading. It would go better if I had more time.

I am doing French and Spanish on here. However I already speak quite good French and some Spanish so I am not starting from scratch. I did French and German at university many years ago so I am used to it. Duolingo copes with it fine.

I am studying Thai and Spanish. Both very different so no problem whatsoever with interference. Thai is difficult as it's tonal so some words have to same sound (to western ears) but differ in meaning tonally (rising tone, low tone, high tone, for example). I already know how to speak some Thai from living there for a while but want to read and write.