Being interested in learning 2 or 3 languages at the same time, I wonder if there are some best strategies to use for better results (regarding time management, things that we should focus on more, etc).
I've tried this myself. My trick is to try to learn a bunch of similar languages at one time.
Among European languages (I haven't done this with Asian or African languages), English, Dutch, German, and the Scandinavian languages are similar. For instance, a certain liquid is "water" in English and Dutch, "Wasser" in German, and "vatten" in the Scandinavian languages.
Another language group is French, Italian, Portuguese and Spanish,(the Latins), in which "water" is eau, aqua, agua, and agua, respectively. In these cases, "if you learn one, you can learn all." This can save a lot of time, if you were planning to study all four languages one by one, anyway.
This can be difficult when learning languages that are not related to each others nor to the languages you already know.
I experienced that myself when I tired to learn Japanese and Kiswahili (an east African language) at the same time as a native speaker of German with knowledge in English, French and a little Bulgarian. No similarities whatsoever. After some time it got me totally confused to the point where I had to drop one language.
In my case I had teachers for both languages but they were different persons and did not train just me. If you had a single teacher for two languages it could be easier.