No, like the others mentioned, unfortunately you can't copy all of your Office11 files because it has to be installed in ordered to run properly, however you can install the files for both partition in the same place and it is going to work fine. Besides it would take up double the space on your hard drive, so instead you can trick the operating systems. So here's what I did... for office 07, and should work pretty much the same for office11 too.
1. I created a Vista partition on my hard drive (before installing vista)
2. Installed Office 07 on the other partition I created for vista
3. Got rid of everything in the vista partition (including the office 07 files) and installed vista on it
4. Installed office 07 on my Vista partition, and the funny thing is that it still works fine on XP.
So pretty much I kinda tricked the operating systems into using the same files, but at the same time also keep the custom settings for each install. Only downside is that you would have to find that install CD or files to do this.