Basically if you are doing sufficient water changes it can be enough to maintain Magnesium alone unlike 2 part where it is required.
I'm don't have a pad and pen to do the calculations right now, but I don't think there should be an appreciable difference in the need to add magnesium between kalk and two part. Both are adding bicarbonate and calcium ions. While 2 part also adds sodium ions, I don't think the dilution of magnesium would be significant, but I haven't done the calculation, so it is possible I am wrong. The largest sink for magnesium thought is its precipitation, and I believe that would happen equally quickly using either method.