I have an external drive formatted for mac. Can I partition that to back up my mac and on the other half, can I back up stuff from the windows operating system?

I also have fusion. How should I go about installing windows? do I just do it through fusion, or should I partition through bootcamp, and have fusion "mesh" (don't know the proper lingo here) with bootcamp to find windows that way.

and will I be able to drag and copy files back and forth through fusion, and also have those files available in the bootcamp version of my hardrive, If i choose to just boot my computer to fun with windows.