All for Reef and similiar products require alk and calcium uptake to be in a pretty consistent ratio of consumption, correct? Some tanks are in this state - others are not. Some younger tanks with decent SPS loads will gradually even out - some never will. More problematic is many reefers assume calcium and alk consumption are in a perfect ratio when they aren't and are stubborn to admit it thinking something is wrong when it's not.
Otherwise you run into the same problem of alk and calcium getting out of whack because you are trying to solve both with one product.
Two parts allow you tweak alk and calcium uptake individually. You are never stuck in the "my alk is low but my calcium is at 500, what should I do" scenario.
"Why not just use kalk given you get a bonus pH boost"
I have a 20gal full of SPS and it would take close to a tablespoon of dry kalk to meet my daily alk needs. I can get away with between 1/2 to a full teaspoon in 24 hours before precip becomes a problem. So, i use a half teaspoon of kalk per day and tweak the rest with two part.