I have always have called it a "stack" switch but I have heard others at the supply house refer it to a "duplex" switch so to help with your problem this means nothing other than you were right,you both are right...
Duplex switch.(where I come from) A device with 2 receptacle outlets is a duplex receptacle. A device with 2 switches is a duplex switch. A device with any combination of devices is a combination sw./rec. or combo. s.p. sw./ 3way sw. or combo. GFCI/sw., etc. What about kits that allow field mounting different combinations of individual devices on a single strap? Stacked?
My argument was that "Duplex" covers fit both the receptacles and the switches, "Decora" covers (or decorator) fit Decora switches and recepacles (and GFCI). Toggle Covers only fit toggle switches (the snap type... not "real" toggle switches that fit into a 3/8" hole, like from radio shack...)
I'm just glad to find out that I'm not the only one in the world calling them duplex switches!
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI