Here's how I would do it:
In the BIOS set the IDE hard drive as the boot drive and install XP on it. After installing XP install Vista to the SATA drive. Vista will replace XP's bootloader with its own on the IDE drive. You'll then be able to boot both from both drives.
Another, less elegant, solution would be to set one of the drives as the boot drive and install Vista on it. Make the other drive the boot drive and install XP. Then change the boot drive in BIOS depending on what OS you want to boot. Like I said this is very inelegant solution; although some motherboards allow you to pick a boot drive without entering the BIOS by pressing a hot key at POST.