That's weird, I tried four or five times and didn't get a single fertilized tile. I don't have any mods installed but I was able to stack units on top of each other. Here is how I did it: Build a unit, move it to an adjacent to the city Build or put another unit in the same city Build a unit in that city, you'll then have two units stacked in the city Moved one of those stacked units out of the city to the adjacent warrior. Of course, once you move one of the stacked units again, you won't be able to stack them. You'll also have to shift+enter to end your turns. When you attack one of these "stacks" the damage goes to one unit until that unit is dead.