Travel to or live in a country where the native people speak more than one foreign language. This is one of the easiest ways to learn more than one language simultaneously. You will naturally try to communicate with the people there. Even if you develop or learn both languages in a transmuted way, your brain will eventually sort out the differences between the dual languages.
Surround yourself with family or friends who will only be speaking in foreign languages with you. Simulate this with friends who are also interested in learning more than one language a time.
Purchase foreign language books in several different tongues; choose books that include grammar and conversational exercises. Sign up for more than one foreign language class. Learn at home in your spare time.
Determine the method of learning more than one language simultaneously that best suits your budget and desire to learn the languages. Set aside funds for your language learning experiences, if needed. Plan well in advance for travel to foreign countries, if that is the option that you choose.
Schedule time on separate days to study each language. Allow a day or two in between each language so that your brain has time to learn, retain information and make distinctions between each language.
Ask questions of foreign language speakers, even in your own language. Determine what words in your language they understand. You may find that some basic words are usually internationally understood, such as "no" and "ok." Use pantomimes, gestures or charades to communicate as needed, if and when possible. Try to take part in conversations. You will learn things just by trying.
Use the language skills that you learn. Practice as often as possible. Look at learning the languages as if they are a fun sport or game.