Yes I've done it myself. There will be an option in the BIOS called "Hard Disk Boot Priority" or "Hard Disk Boot Sequence" or something like that. You can use this to list all your HDs in the order you want them to boot, regardless of their interface. Set them both to master if one is on an IDE channel and one on an SATA channel.
EDIT: You would need to set the the OS that you installed last to be the first HD to boot, i.e. if you install XP onto HD1 then Vista on HD2, you need to set HD2 as your first boot disk.
EP, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS