The duo found the back of the net early in each half as Rennes set up a Stade de France clash with St Etienne after Les Verts beat Lille on penalties following a goalless draw on Tuesday.
The final, featuring two clubs who have never reached that stage in the competition since it started in the 1994/95 season, will be played on April 20.
Midfielder Feret put the hosts ahead in the seventh minute when he dribbled past Vitorino Hilton before beating keeper Laurent Pionnier from close range.
Montpellier came close to an equaliser in the 18th minute but Gaetan Charbonnier's attempt from just inside the box smashed Benoit Costil's post.
Rennes, however, seemed in control and were barely bothered in the first half.
Montpellier looked sharper after the break but Turkey striker Erding added a second for the Brittany side when he was perfectly set up by Jonathan Pitroipa six minutes into the second half.
Rennes then sat back and easily contained Montpellier who gradually ran out of ideas and willpower.