yes, it can done, but microsoft don't recommended it and so am i. there alot of issue in dual OS, such as stability and speed.
For some reason that u still wanna dual OS i recommend u install both OS in separate HDD.
HDD1 = WinXP
HDD2 = Win Vista

During OS installation i recommend onli plug in the 1st HDD in without plug in the 2nd ones. (if ur bios can disable a HDD is much easier)
After installation of the 1st HDD is finish, unplug it then install the 2nd OS on the 2nd HDD without plug in the 1st HDD

After both finish, plug in both HDD, go to bios then set for which HDD to boot
By doing this during OS booting up, it will not prompt for OS selection.
Its much stable this way.

I tried and it work. (better than both OS in same HDD)