Feat: Theurgic Specialist
Source: Dragon #325
Spells from your specialist school benefit from your knowledge of other spellcasting classes.
Prerequisite: Specialist wizard level 3rd, any other spellcasting class level 1st.
Benefit: When casting a spell from your specialist school (off the spell list of any spellcasting class you have), you can add together the caster levels of all your spellcasting classes to determine your caster level. For example, a 3rd-level druid/1st-level sorcerer/3rd-level diviner could cast any spell from the divination school as a 7th-level caster (even when casting a druid or sorcerer spell).
This does not affect your spells per day or spells known. It only increases your caster level when casting spells of the appropriate school