I am doing this course at the moment.
A Joint Honours course is one where you study a mixture of credits from two different degrees, but do less in each. So for example, this year (2nd) I am doing 60 credits from a list for Portuguese, and 40 credits in International Relations, with 20 credits of an elective (French). Your classes are ALL with people doing single honours (though there is no single honours Portuguese degree). It's a good course.
Leeds teaches Portuguese literally from scratch (you start with "hello") but if you want to keep up you'll need to have foreign language skills (understanding the concept of pronouns, conjugation, tenses, etc). Many people on the course already know Spanish, but not all (I know French).
Feel free to ask any more questions about this.