Very interesting thread this one. I did ask this initially but no ones addressed it yet - I'd love to hear what people think. Do you think it's a good idea to learn languages concurrently, that is, to start learning another (in this case related) language whilst your level of the other one is still not up to your aspirations?
European kids manage it. I've taught Germans who have studied English and French or Spanish very successfully. It depends on your aspirations [ie, what level you hope to reach], your learning style and strategies, affective personality factors, whether you're in the target language community, if you're talking speaking only, or want literacy...........
The list goes on, and there are so many variables. Try it and see is the best advice. There's no reason why not as far as I can tell. Pre-schoolers can become multilingual if they are getting the required input. Adults aren't too different, pronunciation aside, and do have cognitive advantages over children.