As the title implies, Multiple Units mod will allow you to place more units on the same tile / spot.
Civilization VI Multiple Unit per Tile Mod
– You can place up to 3 but you can change that by editing GlobalParameters value from the MultiUnit.sql file
Read more here
How to install:
– Download and unzip it to
..\Documents\My Games\Sid Meier's Civilization VI\Mods folder
– Then activate it in Additional Content menu
– Now start a new game.