It depends on what languages you are going to learn and what level you have already got. Languages of common origin like German , French and English or Russian and Ukrainian have similar word roots and Grammar patterns , so it may be easy to learn them simultaneously. But there's a possibility that sometimes you will be confused. I think it depends on your memory and psychological type. If you are going to learn Japanese and Arabic it'll be hard to mix them in your head :) but anyway it'll require huge patience and much time. The Key to the Prevention of Confusion is putting the language into immediate and regular use or practice... However if you put the language into regular practice, for example, with lessons or conversation with a teacher, or by living in a total immersion situation i.e. living in a country where the language is spoken, the correct context, and usage is reinforced. if you learn two languages at at the same time, e.g. you are full-time studying languages, or you go and live in a country/city/town where the two target languages you are learning are spoken e.g. Basel where German and French are spoken, Belgium where Flemish and French are spoken etc, etc... or living near the border of Spain and France, enabling you to commute and converse in different towns... but that is just a broad rule... each person is different, and can cope with different levels of new learning.