I keep various PC fixes,routines,procedures and how to guides that I find useful via the web.You never know when they might come in handy..
I found this possible dual boot setup procedure on the web when I was considering a dual boot.
I have never tried it this way,I have always installed XP first.
I would read the below,and do some investigating first, before you attempt this.
Lets hope some forum gurus out there can comment on this
Disconnect the XP HDD, install vista on the second HDD, then reconnect the XP hdd. Leave XP drive as first boot drive in BIOS. Use the quick boot menu of the BIOS to select vista HDD when you want to boot to that. Check MB manual for quick boot support. I've seen either Esc or F11 at boot to activate the quick boot menu. This way you avoid the vista boot manager, which is somewhat of a pain to get rid of if you decide you don't want to dual boot anymore.