The book specifically mentions that if you have one focus item that adds +2 power to offensive water evocation and a +1 power to offensive water evocation (*note: I don't remember the specific wording - I am paraphrasing) that the item with the larger bonus would be in effect. They would not stack to a +3.
Logically, we can conclude that the book is assuming players will have many, many foci. As such, we can also conclude that a player could have more than one foci to be used for one spell.
There is no rule in the book stating that one spell cannot use more than one foci - just that power and control for more than one item do not stack.
Additionally, one item can have a power bonus for more than one element of evocation. How in the world would that be beneficial unless a caster also had other focus items to supplement that?
I really don't understand why people are even arguing about this.
In the books, Harry only really has refinement foci because he has a buttload of power with very little control. He doesn't need to worry about conviction.
Other wizards (like Elaine) on the other hand have many, many doodads and gizmos to help with spells. Didn't Molly use two wands too?
I see no reason why a spellcaster could not use more than one item. If a GM really wants to nerf casters that badly, they should just run a lower level campaign.
I have looked at "Our World". The author lowballed the stats for most of the NPCs in there, and any one member of the Senior Council could nuke a small country.
It may make some people uncomfortable how much power spell casters can sling around, but that is how the DF universe works....
Sort of like how a Demon with Mythic strength and Claws will be swinging an 8 shift weapon every attack. Without any stresses. In conjunction with their attack.
That level of damage is sick. A demon with that level of power could tear through a tank without too much trouble. If characters are going to survive stuff like that, they are going to have to escalate their power accordingly.
Nerfing PCs is only appropriate if the opposition is not up to match.
When the nearest we have to a beastiary lists NPCs that don't even have stats, just a warning to "run away", I don't think that any nerfing is appropriate.