Multiclass spellcasting

[ShedShadow]

Guys, I have a question. How do you calculate caster level if you have several classes that can cast that particular spell. E.g. I have a lvl 6 character with cleric 3/wizard 3 and I cast the spell summon undead II (Libris Mortis). Both of these classes have acces to that spell, so would I then use the highest caster lvl, three in this case, or the combined caster level since I have 2 classes that have acces to this spell, thus effectively making my Effective Caster Level for this spell 6 to determine the duration of the spell.

Now, say that I had an Evil domain so that I could cast Evil spells at +1 Caster Level would I add that to my cleric Caster Level to determine my Caster Level and would I then use just the cleric Caster levels as my effective caster level to determine duration of the spell, so that my total cleric Caster Level being 4 and the higher one will be the one to use, or is this again stackable with the wizard caster levels so that I have an Effective Caster Level of 7?

Say in this case that I had the Arcane Thesis feat (Player's Handbook II) which would allow me to cast one particular arcane spell at +2 caster level and I would apply it to Summon Undead II, which would make my effective wizard caster level for that spell 5, would that be allowed to apply it to a spell that is both Divine and Arcane since both the cleric and wizard can cast it? Also, would it mean that I had an Effective Caster Level for this spell of 9 (3 cleric +1 domain, 3 wizard 2+thesis) or would I add up the class Caster Levels, being 6 (3+3) and then use the highest modifier available, in this case being +2 from Arcane Thesis? Or would it rather be use the highest caster level available, in this case being my 5 wizard if Arcane Thesis is allowed?

Or, rather than doing all this brainbreaking over this matter, should I use my wizard levels + thesis when casting it as an arcane spell and my cleric levels + domain if I cast it as a divine spell? In the case of casting it as a wizard, does my +1 domain bonus for evil spells still apply?

P.S. do arcane thesis and domain bonuses apply to scrolls and wands etc.?

Hope I didn't break your brains too much ;) thanks in advance

[Waker]

Guys, I have a question. How do you calculate caster level if you have several classes that can cast that particular spell. E.g. I have a lvl 6 character with cleric 3/wizard 3 and I cast the spell summon undead II (Libris Mortis). Both of these classes have acces to that spell, so would I then use the highest caster lvl, three in this case, or the combined caster level since I have 2 classes that have acces to this spell, thus effectively making my Effective Caster Level for this spell 6 to determine the duration of the spell...
When you cast it as an Arcane Spell, you would have caster level 3. Divine, caster level 3. The two wouldn't stack. If you have the Evil Domain, you would cast both at +1, for 4 each. If you have Arcane Thesis, it only affects the caster level when used as an Arcane spell.

So in summation
Wizard- 3 + 1 (Evil) + 2 (Arcane Thesis)= 6
Cleric- 3 + 1 (Evil)= 4

If you wanted to increase your caster level, you might consider the feat Practiced Caster which increases you caster level when taking levels in classes that don't progress spellcasting. Using it in the example above, you could bump one of your classes by +3.

[Yuki Akuma]

You never stack different spellcasting classes' caster levels together.

If you cast Summon Undead II from a Wizard spell slot, you cast it with a caster level of 3. If you cast Summon Undead II from a Cleric spell slot, you cast it with a caster level of 3.

Your domain ability counts whenever you cast an [Evil] spell. So you'd cast an [Evil] spell as a Wizard at CL 4 and as a Cleric at CL 4.

Arcane Thesis applies to arcane spells. Clerics do not cast arcane spells. Wizards cast arcane spells. So when you cast this particular spell as a Wizard you have a CL of 6. As a Cleric, still 4.

Arcane Thesis and the Evil domain ability don't apply to scrolls and wands - they have a fixed caster level. I'm unsure if it would apply to staffs.

[ShedShadow]

Thanks guys :) It helps a lot.