https://wikicarpedia.com/index.php/File:Farmers_example_b.png
In the above picture, the top tile on the left has a red knight and a yellow farmer on the same tile. With the exception of a flier or a Phantom, this doesn't seem possible to get two different players' meeples on the same tile. Thoughts?
You're right, having more than one meeple on a single tile is possible only with an expansion :
A flier (note that fliers cannot be used to place farmers), a crop circle, a magic portal, the plague (if a meeple is moved to flee the plague), a phantom, or if one of them was a wagon (note that wagons cannot be placed as farmers).
The city of Carcassonne may also allow more than one meeple on a single tile, but not at the step shown in the picture.