It's possible to double-stack crops using planter boxes, particularly useful for crops that don't require hydroponics. It just depends on the order you place the objects.
Create two farm tiles. In one, plant your crop of choice, say
mushroom MealLice. Now, try to place planter boxes. You can place it on top of the unplanted tile, but not the planted one (which is why I'm assuming this is an exploit). Now, with your planter box built, plant yourself two mushrooms MealLice in the same space, one in the box, one in the farm. Both will grow correctly.
EDIT: I was wrong about the mushrooms. So far I have been only able to do this with Meal Lice. Sorry about that.
Steps to Reproduce
See description of exploit above. Create a farm tile, don't plant. Build planter box on top of it. Now plant in both.
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.