Right, here's a slightly more technical explanation. Emulators for the DS have two options. They can either take advantage of the DS's fast dedicated 2D hardware for drawing to the screen, using DS sprites and tiles (this is called hardware rendering), or they can function like emulators for the PC operate, that is, figure out what the screen should look like in memory and then paint the image to the screen pixel by pixel (called software rendering).
Hardware rendering is faster, and due to this jEnesisDS uses hardware rendering, but because of the way the DS renders the screen you can only scale vertically, not horizontally. The reason is because the DS draws row-by-row, so you can simply skip a few rows here and there to squish the screen vertically. You cannot however skip a column of pixels when drawing. Thus, horizontal scaling is not possible with hardware emulation.
With software emulation, since the emulator determines what the screen should look like before painting it to the screen, the image can be modified before being drawn to the screen, thus allowing horizontal scaling to be possible. This is unlike hardware emulation in which the DS doesn't really know what it's displaying on the screen, it's just blindly drawing it (not really "blindly" but more blind than software emulation at least).
jEnesisDS v0.4a has a software renderer version, as does PicoDriveDS, but because software rendering is much slower, you'll have to use frameskip to maintain full speed, and you won't have any sound.
