Consider a scenario in which two people who do not speak L2 try to learn it without human resources. All they have are books in the language, dictionaries, and online learning tools like Duolingo. What advantages to this strategy are there compared to learning by yourself? Are there any disadvantages?
Comparing learning with another non-native speaker vs. by yourself:
- Able to practice speaking together in the language.
- You can help each other to stay focused and keep learning for a long time (this depends on personality of course).
- If you have trouble understanding a grammatical concept, your learning partner can help you with it, and vice versa. In the case that both of you are struggling with something, you can discuss it and figure it out together. Two heads are better than one.
- If you are speaking to each other in the language, you may learn each other's errors, or reinforce each other's errors, and these can fossilize. This can be especially troublesome with "calque errors" - where you translate a phrase or grammar pattern literally. You may not realize that native speakers of the language don't say it like that, so you keep using it.
- You'll probably adopt a pronunciation based on the phonemes of your own language. For example, if two English speakers are learning French, they may use the English rhotic sound [ɹ] instead of the French one [ʁ]. This can become fossilized.
However, the biggest disadvantage to both of these methods is the lack of interaction with fluent speakers. If at all possible, it's best to get some interaction with fluent speakers (even if non-native). Failing that, both learners should get as much input from fluent speakers, especially in the form of audio / video, such as movies, TV, videos on Youtube, etc.