It's a program which basically allows you to have multiple instances of Minecraft, so you don't have to remove all the mods and reinstall to change modpack, in almost the exact same way that the Technic Launcher does.
It may be possible to have more than one MultiMC running at a time, because if you put different instances of the same thing, the programs won't be modifying the same files. You'd just have to copy the TekkitSMP folder into instances, I think.
http://www.minecraftforum.net/topic/1000645-multimc-42-windows-linux-mac/