Dual Boot question and software Options?

[RonH54]

I have XP Pro on my "Main H.D" and when I installed Vista Build 6000 on a separate H.D. I plugged my "Main" H/D.,installed Vista Build 6000 on secondary H.D. because I wanted to pretty much wanted Vista not see my XP when I installed. I now would like to install a 3rd party Boot Manager on either XP's H.D. or Vista's H.D. Is they a Dual Boot program you can Dual Boot if the O.S. programs are on different H.D.'s not different partitions of same H.D.?

Thanks