Two rules go against this. First, the description of a Stacked Focus (underlining for emphasis):
Stacked Foci
It is possible to combine two or more types of foci into a “stacked” focus. The effective Force of the focus is equal to the sum of the ratings of the different foci.
Second, and possibly more important is the fact that while a Stacked Focus lets you combine multiple Foci into one object, it does not increase the actual force of the individual parts. A stacked Force 2 Weapon, Force 3 Sustaining Focus does not function as Force 5 when Sustaining Spells or adding to attacks with the weapon. You use the effective Force for determining the threshold of the creation test and the maximum force of bound foci, etc.
Now, two Force 2 Weapon Foci stacked together would still function as two separate bound foci for the purposes of what they add. This means they would each add 2 dice to a test. However, you cannot add bonuses from more than one focus to any single die pool. So using two Weapon Foci would be ineffective at best.
Regardless of the number of foci a magician may possess, only one focus may add its Force to any single dice pool.
