I have noticed that just recently (for the last several weeks), I am no longer able to cast this on myself when I have O'keil's Levity on. When I try, I get the message "your spell did not take hold."
The O'keils levity damage shield is only 5 points of damage, which we could double by casting O'keils Flickering flame to get a "not too shabby" damage shield (10 points total) though not nearly as good as druids or rangers, hell, not even as good as necros (theirs is 12 points). Now, we're back down to 5 points on the damage shield, max.
If anyone can get both spells active on the same character now, please post.