Install XP on one disk (you might have that already). Then install Vista on the other disk and tell the setup that you want a dual-boot configuration.
You'll find a lot of information in http://www.google.com/search?q=xp+vista+dual+boot
Of course you realize a disk must have a partition (at least one) to be usable. So I interpreted your requitement as "one partition on each disk" (in stead of "a hard disk with no partition").
I need help o dual boot from xp and vista, which will be on seperate hard disks with no partitions at all. XP will be my primary OS and Vista will be there only for media centre, but still, I don't want to partition anything at all.