Can a character use spell slots from one class to activate a Runestaff spell that's on the spell list of another of their classes?

Question

A runestaff (MIC p. 223) allows a character who's attuned to it to sacrifice one of their spell slots to cast a spell of equal or lower level from the staff. This allows characters to cast spells that they don't know.

However, the spell must be on the user's class spell list. From the item description, MIC p. 224:

By expending a prepared arcane spell or arcane spell slot, the wielder can cast a spell of the same level or lower from the runestaff’s list, as long as that spell also appears on the wielder’s class spell list.

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If a character has more than one arcane spellcasting class (say, a Duskblade/Wizard) can they expend a spell slot from one class (say, a Duskblade slot) to cast a spell that appears exclusively on the list of their other class (say, Wraithstrike)?

Answer 1

Yes

The requirements listed are:

• The character must be attuned to the runestaff in question.
• Sacrifice an arcane spell or spell slot of the same or higher level than the target spell in the runestaff.
• The spell appears on the character's class spell list.
• The spell also appears in the runestaff's list.

Note that you do not have to currently know the spell, it simply needs to be on one of your spell lists.

Note also that it does not restrict the spell or spell slot to any one class, it simply has to be arcane.

Here are more details about runestaffs.

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It is worth noting that there does not seem to exist much in the way of official clarification regarding using multiple different sources (specifically differing spell lists from multiple classes or prestige) to activate or use an item. This is more of a case where it is assumed that if one meets the requirements, no matter how one did so, it works. On the other hand, there doesn't seem to be any specific indication that it wouldn't work either. There are magic items that specifically require multiple sources to create, so it's not too much of a stretch to have multiple activation sources either.

Therefore this answer assumes a binary test of "do you possess requirements 'X', 'Y', and 'Z'?" If so, then it works, if not, then it doesn't.