[GaidinBDJ]

Ok, well, this is basically at your own risk, but here's what I did. It assumes Win7 (Back up everything first)

I used Win7 simlinks to point %APPDATA%/.minecraft to two different "real" folders. For example, my setup currently has c:\minecraft\minecraft181\ and c:\minecraft\minecraft19pre4\. The contents of my %APPDATA%\.minecraft for each respective version was copied over. (So I have currently two full Minecraft installs, one for each version)

Then you can simlink to whatever folder you want to run using the command:

mklink /d %APPDATA%\.minecraft c:\minecraft\minecraft18

That will create a directory simlink. You can make a bat file to delete your current .minecraft simlink with the command:

rmdir %APPDATA%\.minecraft

Then just add Minecraft.exe to the bat file (assuming your minecraft executable is in the same folder). I have two bat files, one for 1.8 and one for 1.9 pre4 (and two different directories in c:\minecraft)

I'm sure someone can take this idea and state it a bit more eloquently. I'm good at doing, not so much with the explaining.