First of all, two part dosing is directly adding Calcium and Alkalinity into the tank, the result is immediate.
Like Coralfishreef said, AFR requires bacteria to transform the AFR into alkalinity, depending on the amount you need to dose, and the density of certain bacteria in the tank that transform the AFR into carbon dioxide, the alkalinity can potentially increase slowly overtime due to slower biological process. For nutrients rich tanks, it does not seem to be an issue, possibly due to the higher bacteria density in the water.
another potential issue, which may not be an issue, is that little is known about what formate can do to coral and fish. AFR is calcium formate.