I don't quiet 100% agree.

Learning Swedish, Danish and Norwegian does not necessarily mean mixing the languages. The sound is very different and to separate specific vocabulary is possible. Same with Spanish and Italian. The music of the languages is very different, so unless your goal is to get a native level of pronunciation it should be manageable to have progress in both. I agree, that it is a good idea to focus at a primary learning language when you try to learn languages from the same language family at the same time.

Just for curiosity, which are the two languages that you have learnt at the same time? And how would you describe the particular challenge of that particular language pair?