How do I create a plant that is two blocks tall when placed, like a rose bush, lilac, etc?

This is being planned to be made simpler in the next update, but here's how to do it for now:

Create two different blocks: the top texture, and the bottom.

Make it so the bottom can only be on a certain block, if that is what you want.  

Make the bottom block be the ONLY one that drops your item.  

Make it so when either block is broken, the other breaks along with it.  Make sure the  bottom block drops the item.  

Make it so the item places the bottom block, but when the bottom block is placed, the top one will be placed as well. 

I believe that is all you need to make it.  Good luck!

Edit: make sure the top block is not in the creative inventory.  the bottom will place the top, so you don't want to be able to place just the top half of the plant.  

you might not need it anymore, but there is a more in-depth tutorial here: https://mcreator.net/forum/63968/tutorial-tall-plant