I was looking into a dual JD/MSW degree, but I've (tentatively) decided that it really doesn't justify the extra cost. I figured the education and social work background might make me a better lawyer for some family law stuff, but so far I've yet to hear how it can actually bring more money my way. Not that it's all about the money for me, but investing an extra $30k or so for something that *might* make me better at a legal field just didn't seem wise.
For what it's worth, I've had one school tell me that joint degree is something different that dual degree, but I haven't actually figured out what that difference is.