Thank you Turbine devs for making Spell Focus (previously called Spell Focus Mastery) stack with specific school items. This is a step in the right direction for making DC casting effective again!
"Spell Focus Mastery +2: All spells have the DC to resist their effects increased by +2. This bonus does not stack with other item-based spell focus effects."
"Spell Focus II: Passive: +2 Equipment Bonus to the DC to resist all of your spells."
Note that Turbine explicitly removed the phrase "This bonus does not stack with other item based spell-focus effects."
Thanks for finally providing a reason to equip Stormreaver's Tablecloth and Twilight. It's about time that spellcasters got a the same treatment as melee who have long benefited from Combat Mastery and a specific tactics bonus (such as Stunning +10) stacking with each other.