A long time back, KI staff posted that it was fine to stack, so people do.
But I think if we could buy a stacked bed, it would sell.
There should be several stacked beds - one for large, one medium, one small. And because the spells will only effect a certain number of plants, there should be enough plots on them for the max the spells can effect. Also, it shouldn't be higher than the max a spell can effect without needing a crate or something to balance the spell on.
Then it should be available in the crown shop for like 10k or more crowns.