As you surmised, two different items can't be built in the same square. So the regular planter bases and the regular trees can't both be on the same spot.

But, if you look in the Decorations toolbar, all the way to the right, there's a button called Ghost Decorations. Things in this toolbar can be built *anywhere*, even inside another building.

So either the planter base, or the decorative tree need to be built from the Ghost toolbar, and then they can be put on the same square.