I'm actually a double major in JO and Anthropology and I have a friend who is a double major in French and JO. I'm not sure just how many students are double majors, but I do know that all JO majors are required to have at least one minor alongside their major.
But it's definitely possible to double major with JO and still graduate in 4 years, I mean I've been able to do it and I even studied abroad last semester. It does require a little planning ahead of time, going thru and figuring out what classes are required for each major - but a lot of students are able to do across all majors.
Hope that helps!!! Don't hesitate to ask any other questions!!