this can be done without much problem but you can't see both server catalog at the same time. You can see only 1 at a time.
You have to just rename the catalog folders
1> if you want to see the 2003 catalog then copy this folder from xp machine and copy it to c:\program files\symantec \backup exec location of ws 2008 server. Make sure you rename the existing one to catalog.lto5 or anything that suits you.
2> You can also change the default location from tools - options - catalogs
change the catalog location to any location whenever you have the catalog folder.
But this way you can see only 1 machine catalog at a time and i don't think that will be a problem as you will be only accessing it when you need 2003.
Make sure you have to restart the BE services after doing this.