Anyone who can teach one language could easily teach two.
There are jobs in which you need strong determination and talents and I believe that being a teacher is one of them. Not only do you have to possess broad knowledge of the subject but also such features of character that will help you to supervise and have control of your students. I believe that if you have those characteristics you can teach one or more foreign languages providing that they are to some extent similar to each other.
First of all, a person who knows and is able to teach vocabulary, grammar and communication in one language, can use similar techniques to explain students the rules of another one. You can teach, for example, English and German, German and Swedish, Spanish and Italian and so on. Anyway, it is not recommended to teach or learn such completely different languages as English and Chinese, German and Polish or English and Urdu as the student may feel confused and will not learn any of them properly.
Once you are a teacher of one language and you have been successful in teaching it for some time, you can be sure that you possess the skills and experience to try and teach another one. You will probably not repeat the mistakes you made at the beginning of your teaching practice.
We are all aware that people who know one language have few difficulties in studying the second or third one. As far as teaching is concerned, the situation is similar. When you already know how to treat your students and how to prepare the material or organise the lesson, the only issue to be considered is if the differences between the two languages will enable you to use similar teaching techniques.
Another thing that is worth mentioning is that it is easier to teach students who already know one foreign language as they have some linguistic language that gives them the basis for effective studies.
All in all, I think that any teacher who is considering teaching two languages should not hesitate as he or she has the appropriate skills and experience to do so.