My understanding for focus item stacking is that there are two aspects to the item.
A) The element(s) affected (i.e. the traditional elements Fire, Wind, Earth, Water, Spirit)
B) The total bonuses given (Offensive Power, Offensive Control, Defensive Power, Defensive Control)
Add up the number of type A and type B, multiply them, and that's how many slots it takes to create. So a Fire,Wind,Water +2 Offensive Power, +1 Offensive Control focus would be A=3 Elements B=3 bonuses. 3*3 = 9 focus slots.
This seems to fit with the examples in the book. It would take the exact same number of slots to instead create three separate focuses for each element that provide the same bonuses to each, adding them all into one item is just a way to avoid needing a dozen different focuses depending on the element and what kind of bonuses you want, the Lore limitation prevents it from getting too silly.
I'm somewhat confused because some posters here seem to be implying that there is some inherent numbers advantage to splitting vs. combining your focus item slots and as far as I can tell there isn't (besides putting all of your power into one item and being separated from it). As to examples of a +5 Fire Offensive Power Gauntlet being broken... well, yea it's powerful but there is a reason it's able to stack directly like that instead of the +1, +2, etc. requirement that skills have. The reason being that it only affects one facet of one type of your evocations. Specializations affect your Power/Control directly for the elements and are subject to the stacking rules (you need a +1 in order to have a +2). Focus items cost exactly the same amount of Refresh as a specialization does, so without this inherent stacking bonus there is no point in getting them over a specialization.
Edit: Oops, now I see. They're talking about getting past the 'no stacking bonuses' rule by putting offensive Power on one focus and offensive control on another, thereby allowing a +5 to both. Yea, that is a nasty combo :p