You can place banners on top of each other or on top.

I expected it to not place or be placed next to.

I made a orange banner in creative mode using a crafting table. When placing them down I saw the hit boxes around the banners and found that you can place the newly placed banners on top or almost attached hovering.

Steps to Reproduce:
1. Make banner (orange for example)
2. Place banner down
3. Write click on banner hit box area
4. Banner should appear hovering or attached.