Hey, I think I can give you a bit of advice since we are in the same boat only that I am applying for the JD/MBA concurrently (and I'm also applying for UVA law/Darden). The LSAT is a significantly more difficult test to master than the GMAT (there are no pesky Logic Games on the GMAT for one). To give you an idea, I've been studying for the LSAT since June. I've raised my score from a 163 to a 173 and am hitting it hard in the last week to keep it at that level. In contrast, I began studying for the GMAT in late-July. I scored a 640 on my first diagnostic. I took the GMAT on Aug. 23 and scored a 710.
To study, I would recommend buying the Official GMAT Prep Guide. This is the one with the orange cover and has just tons of real GMAT questions in it. Also, download the two official tests off of the
https://www.mba.com . Take one in the beginning to see where you stand. Do a bunch of questions (the harder ones are in the back of the sections in the book). Then take the second online practice test and see where you stand. If necessary buy the supplementary books for Verbal and Quant also written by the makers of the GMAT. I would avoid anything by Princeton Review, Kaplan, etc. They don't use real GMAT questions and, to me, they do not feel like the real test. Also, their diagnostics rate you much lower (so that you always score higher on the real thing and can never get a refund). Some people like this, but it almost shot my confidence. Stick with the official stuff and see if the material that they give you is enough for you to crack 700. If you crack 700 and you are already in UVA law, I'm pretty sure Darden will be yours as well.
Good luck on the JD/MBA, maybe I'll see you in Charlottesville next year!