Choose two languages that are not from the same language family. According to Dr Mikihiro Moriyama of Nanzan University, languages that are closely related can be confusing to learn at the same time because the vocabulary and grammatical structures are so similar. If you choose two very different languages, such as German and Arabic, you'll have a much easier time keeping them separate in your mind.
Start studying one language several weeks or months before starting the second language. Once you have the basics down for the first language, you will feel more comfortable starting the second language. Researchers at Nanzan University found that students acquire languages more quickly after having begun studying one foreign language.
Keep a written record of vocabulary learned from each language, and review this record regularly. You can make flashcards out of 3 x 5 cards and store the flashcards for each language in a separate card file. Or keep a spiral notebook for each language, and write down new vocabulary words in each notebook as you learn them. Regular review will keep the words fresh in your mind and help you to avoid confusion.
Speak with native speakers of both languages. In order to pick up on the nuances and correct pronunciation of both languages, you'll need to listen to native speakers and practice speaking to them. If you find yourself allowing one language to influence the other, make a point of listening carefully to native speakers and asking for feedback on how you're doing with your pronunciation.
Learn about the countries where each language is spoken. As you learn about the cultures, history, and people involved with a language, the idioms and expressions common to the language make more sense and feel more natural in everyday speech. You'll also be able to keep the two languages separated in your mind better when you associate each language with a distinct culture and people.