Door merging / Door bridge

By placing 2 doors inside of each other (1 unit at the top and right side of each door is transparent in collision when editing) or by using Blueprint Desync.
You create a door that is sort of merged.

You can use this to create "Door bridges", which is the official name for these.
However, there have been people who have done insane things with these,
like giant garage doors, door stairs, even entire traps designed with them.

So it's suffice to say this is one of the endgame mechanics that players with nothing else to do might want to experiment with.