What form of magnesium do you take? Certain forms are more likely to induce loose stools than others.
One way to increase bowel tolerance is to take a fiber supplement apart from your vitamin C. Fiber supplements, like psyllium, can help to add more bulk than your diet will allow.
The key is to take the fiber in between meals - not with your other supplements. For instance, if you eat breakfast (and take supplements) at 8:00 am. You can take the fiber at 10:30 am. Then, have your lunch at 1:00 pm.
The ideal would be to take your fiber with at least a two-hour window between meals (more if you can). And, make sure to drink plenty of water with your fiber.
I've found that encapsulated fiber works better (for this purpose) than the powders. This is the product I most often use:http://www.iherb.com/ProductDetails.aspx?c=1&pid=752
I need to take considerably more than the recommended dosage for the desired effect. This isn't a problem, both safety-wise and because I can swallow many pills easily.