The following gif shows two color tiles stacked into the negative.
When several items are placed on rollers, they can be stacked normally.
In order to stack in the negative direction, the stack tile must be placed in the roller. Attention: The roller cannot be placed on the stack tile, therefore the stack tile must be placed in the roller. The stack tile must also not be on top of the roller! Now the color tiles are placed in the stack tile.
The color tiles are now stacked into the negative. However, you can see that the yellow color tile does not continue into the negative but is stacked into the blue color tile. Therefore, two pieces of furniture must be used by placing the top one back into the stack tile.
Thus, furni can be stacked into the negative. However, there must always be at least one person in the room to keep the furni stacked in place. Unfortunately, during a full room reload (when you enter a room with no Habbo in it), the furni is brought back to normal floor level.