Ok then they are doing the Balling.
There's no perfect way to supplement calcium and alkalinity, as all systems have one or more drawbacks. Folks just have to decide what systems seem best within their budget, their tank expectations, and tank size.
When Bob Stark of ESV invented the two part, the trick was to put those same required ions that are in Balling Part C into the two parts in a way that was stable, making it easier for users (2 things to dose, not 3) . It's probably not exactly the same ingredients, but it is pretty close, and I agree that functionally, at least in theory, they are the same. Many two part systems now follow the general ESV design, though there is not any analytical data to indicated how good any of them are.
If folks are using my two part, I recommend using my part three as I described (which is not based on magnesium measurement), or preferably, use the Balling Part C instead of my part 3. BRS suggests this and I think it an excellent idea, although they seem to get the implementation a bit confused by suggesting using both my part 3 and Balling Part C, which duplicate each other.
I was very clear in designing the DIY two part on what was balanced and what was not. The major ions (sodium, chloride calcium, magnesium, sulfate, potassium and bicarbonate/carbonate) were balanced in my original recipe. Other ions (e.g., fluoride, etc.) were not, and I agree that isn't perfect. Water changes help that issue.
Keeping ions in check is why I recommend water changes for anyone using any method short of frequent ICP measurement of all important ions (and even then I think it useful for dealing with organics).
