Here is a save file that shows that the fortune teller ability does indeed stack.
I have set up 10 provinces for LA Ulm, and there are ten Fortune Tellers (who have 'Fortune Teller 10' ability) in each province, giving the obvious combined total of 100 Fortune Teller ability in each of the ten provinces. Each province should also have Turmoil 3 Misfortune 3 from the Pretender scales, designed to maximise the possibility of bad events happening.
If you click end turn a few times (apols for the lack of cash, Jomon, and the slightly lengthy turn process time) you should find that no bad events appear in any of your provinces.
Although it is worth noting that no amount of Fortune Teller ability will actually increase the likelyhood of good events happening. Misfortune3 still means there is a -39% chance of a good event happening, and even a combined Fortune Teller ability of 100 does not change this percentage chance at all. All it does is give a percentage chance to nullify a bad event should one be triggered to happen in a province. Which is more or less exactly what the description of the Fortune Teller ability says
Edit: Guess I should probably dig up a more relevant thread on the Fortune Teller ability for this post than one to do with an undead commander.