How serious is it on small twin engines with only one door? Or with 2
doors, for that matter?

A friend of mine tells me that he once had a twin engine Piper crash to
investigate due to ditching. It was later learned upon the conclusion of
the investigation that a passenger in the back seat was trying to switch
places with someone in the front and the door inadvertently popped open. At
that point the investigator determined, from his own similar experience,
that the plane sunk like a rock due to critical disruption of the airflow
to that could not be corrected in flight.
--The door could not be closed again!---

As for my friend with his similar experience, his friend's hand was all
bloody from trying to hold it closed as much as he could. Fortunately, they
made a safe emergency landing at an island the just happened to be nearby.

I've only flow small single engines and had NO IDEA how serious this could
be. They don't teach you that in flight school. Or is it because single
engines with only one door do not react the same as the twins.

Tell me the straight skinny so I know next time I go flying.

Thanks