*bangs head against table* This should really be up in the same sticky as "GBA emulation on the DS is not possible" given how many times it's been answered, but whatever.
There is no way to do this without severely slowing down everything. If you want to be able to squish the whole screen, use jEnesisDS v0.4a, "SW" version. Every other version of jEnesisDS uses the DS's hardware to make games run at full speed, which requires a few sacrifices, like the ability to squish the screen horizontally.
The latest version has a sort of auto-follow thing where it will move the screen when you press left or right, but I don't know how well it works.