You can stack items by editing their coordinates in game data (which you can find in tools > game > data) by changing x and y values to be an exact match, example:
ITM 1 1,6
ITM 0 1,6
The item with the lower ID takes rendering priority and is the only one that appears visually, all other items share its layout. However, each item still keeps its own dialog definition if you’ve assigned unique dialog.