I may be wrong, but I think the problem is that "spell" isn't what's it's doing, it's a label for what action to take. Try this...
Code:
ExecuteWarning_Pet_BuffFrame:SetAttribute('type1', 'leftSpell')
ExecuteWarning_Pet_BuffFrame:SetAttribute('leftSpell', "Health Funnel")
ExecuteWarning_Pet_BuffFrame:SetAttribute('type2', 'rightSpell')
ExecuteWarning_Pet_BuffFrame:SetAttribute('rightSpell', "Life Tap")
I caution I may be misunderstanding how that mechanism works, myself.