Because Magnesium and Calcium are both divalent the primary mechanisms of absorption tend to compete if you take them both together. You'll get higher bio-availability if you separate them in time (e.g. one with breakfast, the other with dinner).
Thanks for the suggestion. I think my problem is I'm just not getting enough calcium now that I'm getting a lot more magnesium. My legs and arms occasionally have been twitching and I've been having terrible sleep starts, since I started taking magnesium.