Well I actually ran across this the other day called Dock-It. It allows you to create as many extra docks as you want, though I do have to say though it isn't quite the same as they look slightly different than the real dock, it does give the desired effect though. It's shareware but does offer a fully functioning demo.
There's also a program called DockFun!...never used it myself, but it looks like what you want. It can have multiple "Apple" docks that only appear one at a time, or it can create multiple instances of its own dock to supplement the built-in one...that's probably what you're looking for.