I was wondering if a Multiclass elf, Figher/theif, would be able to get both Nalia's castle and the thief's guild, or would they only be able to get one, or neither?

Thanks for your help

You pc can get either the fighter stronghold or the thief stronghold. In thoery you are only suppose to have ONE stronghold, so you pick either one of these.
Although you can have 2 strongholds through loop holes(not sure if the patch have fixed this):
1) Free Nalia's keep but do not accept, then do the thief's stronghold quests (Renal), go back and tell Nalia that you reconsidered and wants the keep; done this but it was before I installed the patch;
2) Someone has mentioned that he was able to obtain the fighter's stronghold despite having completed the thief's stronghold related quests first. Have not tried this one myself.

------------------
Long live H22A1!!!!!

A wizard multi/dualclass can get the planar sphere and the other class's stronghold as well.