Yes. In fact, for some people it may even be a good thing and help learn the language. The mind is like a muscle and can so exercise a lot better if it needs to 'stretch' itself in different directions.
However, make sure it is not two languages that are related to each other. Say Spanish and Portuguese or Dutch and German. This is because the words and grammars are at time very similar and so it is easier to confuse the two and make the lessons harder. Some feel that the similarity in grammar may actually help to pick both up faster, but it really depends on the student.
Best is to choose a language combination that is so different, your mind will need to work harder on it: Japanese and German for example, or Spanish and American Sign Language.