A true two part, either my DIY or a commercial one such as ESV's B-ionic has three main features:
1. The liquids are exactly balanced so if you add 100 mL (or any equal volume) of each part, then when corals use the alkalinity that was added, they also use exactly the amount of calcium added. Thus you do not need to check calcium frequently at all. All of the dosing can be done off the alkalinity.
2. When corals use the calcium and alkalinity, they also use magnesium. The product will contain this appropriate amount of magnesium, and youmay never need to measure or supplement magnesium.
3. After corals use the calcium and alkalinity in the supplements, what remains is an assortment of salts that has the same composition as normal seawater, so the chemistry of the aquarium does not get messed up over time by too much addition of sodium (from sodium carbonate/bicarbonate) which is the usual alkalinity additive, and not too much chloride, which is the usual counterion in calcium supplements.