Kdenlive/Manual/Effects/Audio/Mono to stereo
Mono to stereo
There is a small bug in Kdenlive that makes this mono to stereo effect confusing (in 0.9.4). In 0.9.4 you should set both "To" parameters to the track you want to copy to and "From" as the track you want to copy from. The swap option exchanges the two tracks instead of copying one onto the other.
Keep in mind that this effect (name is probably misleading) is not for mono clips. This "Mono to Stereo" effect is designed for clips that have 2 or more audio tracks, and allows you to duplicate one of the tracks on the other. For example if your left channel is bad for some reason, this will allow you to have the right channel copied onto the left one.
Source: this forum post by J-B-M.