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").

Kees