I know I'm a tad bit late...but I just recently found out about LSD.
On joint law and doctoral studies...There are a number of existing joint programs out there. You just have to locate them. For instance, NYU has a joint law and society program (approx 7 years to complete). Additionally, most law schools mention on their websites that they will allow for an individualized joint program of study. This is the approach I took, and will begin JD/PhD studies in 2008. (As a sidenote, I did learn through my research that, although schools tell you such an individualized arrangement is possible, it is a lot of work on their end, and many will just prefer to not work to create a new program just for one person. These programs require approval from academic deans from many university departments, and many other types of exceptions and approvals. Of the eight schools that I proferred my joint program to, six flat out said no, one was wishey-washey, and one was excited about it and seemed willing to work with me to explore further. I decided to go with this school).
To make such a curriculum work, I would think that your intellectual interests should largely overlap both fields. You mention trial and criminal defense on the law side, and then international relations on the poli sci side. At surface level (at least to me) those don't seem to intersect. Lack of intersection is a hard sell. I make this point because it's important to realize how 'tough' it is to get into PhD school. The law school that I'm attending has an acceptance rate of about 20% or so; the doctoral program I'm going to admits a much smaller percentage (I estimated between 5% and 10%; although, no data are made public). On average, the PhD programs I considered only bring in 2-3 new students each fall.
Aside from internally figuring out whether or not this is something that you really want to do, I would advise talking/meeting with JD/PhDs who are in the position that you see yourself in after everything is said and done. I found this to be extremely insightful. Also talk to current joint program students about their experiences. Hope this helps.