I think you're right, in a test tube it would. But in larger volumes of water I know the chemistry works differently, else no marine salt mix would contain magnesium and calcium carbonate in the bucket, but the better mixes certainly do. Also, dosing in two parts would be in vain, for as soon as the dose containing magnesium mixes with the one containing calcium, the magnesium would become useless for coral uptake, as it would precipitate to the substrate. So something else is going on besides immediate recombination into a precipitate. The chemistry of a reef tank is complex, and further complicated by living organisms doing their things. Best to experiment a bit and gauge any differences. I do notice a difference in my zoathids after adding iodine to my Kalkwasser.