fall between (two things)
To drop between two things (specified after "between"). The baby's crying because her toy fell between the couch cushions. Uh-oh, that letter just fell between the wall and the radiator, where we can't reach it.
fall between (something and something else)
to drop between things. My loose change fell between the cushions of the sofa. I hope that my wallet doesn't fall between the table and the wall. I would never be able to move the table to get it.