it is very possible
unless you have some hardware that you absolutely need, and there are no x64 drivers for it, i don't see why you would need to do this.
First, you have to make sure you have 2 separate copies of the OS. you can only have one version installed @ a time.
It should be installed the same way you install 2 operating systems on any computer.
you need to create a separate partition for each OS and install them independently.
There are allot of great tutorials on how to install 2 Operating Systems