Can you learn two languages at the same time? And if so, how?
Spread your focus (but not 50/50)
If you learn two languages at the same time as an adult, it is better to actively focus on one of the two languages. Suppose you learn French and Dutch, then you pay attention, for example, to expanding your vocabulary and learning the grammar of Dutch, while maintaining French.
Maintaining means that you are busy with the language, but not in a very active way. You could watch television in French or occasionally read an article in French. But the vast majority of your focus is on Dutch.
Make sure there are differences
It is better to learn two very different languages at the same time than two languages that are similar. You can learn Japanese without any problems and at the same time be busy with German, because those languages are very different. Mastering German and Dutch at the same time is another thing. That is a lot harder. Because the languages are so similar, chances are that you will confuse them.
It also helps if the level at which you master the languages differs. If your German is already much better than your Dutch, you could learn them at the same time. In fact, the overlap between the languages can even speed up your process.
In Dutch we say: wie op twee hazen jaagt, vangt geen enkele (he who hunts two hares does not catch any). In short, it is better to focus on one language, because you will achieve success faster and success will keep you motivated. Although it is certainly possible to learn two languages at the same time, it is not advisable. If you do it, be aware that the learning process is likely to take a little longer. That prevents demotivation!