Polyglot Luca Lampariello answers the question “Is it possible to learn more than one language at the same time, and if so, how should I go about doing it?” in a 13:34 minute video and accompanying blog post.
I think he hits the nail on the head that for most people two is the practical limit to learning languages effectively for most people because who has time to get a good base (net) with four or five languages at the same time?
I'm not afraid of learning working on two languages at the same time. I've been working on Chinese and Russian. They're different enough I'm not confused. At least not yet. I'm not really worried about overcoming challenges I may run across because I'm interested enough that I'll keep plugging away. If I waste a little time, I'm not really in that much of a hurry, I'll have had fun along the way so it won't matter to me.
But along these lines, there is one thing I've noticed when I'm studying Russian. Spanish is my second language, and sometimes Spanish sometimes seeps into my mind when I'm working on Russian. I think it because Russian and Spanish have some sounds in common, and since I knew Spanish already my brain was finding it a little easier to relate to the Russian sounds from Spanish than from English.
So, my question for a polyglot might be ... do you work on a third language from a second? Or do you always work on languages from your native tongue?
I like to watch videos on YouTube that Russian speakers have created teaching Mandarin words and phrases because I find it easier to read the Russian alphabet then Pinyin at this stage in my learning. So far isn't confusing me to think of connecting new Mandarin words to both Russian and English equivalent. I would be curious to know if others find it useful or not so helpful.