i've never needed or implemented a 2 os system with a separate os on a separate physical drive. i think that would be a minor problem, as your pc has a boot sequence as to which drive to boot first. so, let's say drive c: has vista and drive d: has mint.
unless there's another way around this that i'm unaware of, you'd need to change the boot sequence and make the os drive you wanna boot at the top of the boot list. you'd do this as soon as your pc starts booting by going into the bios control (usually by pressing the 'delete' key).
better to just use one drive for all booting. never had a problem with dual booting and have been doing so for over a decade. on a typical 160-300 gig hard drive, each os will eat up maybe 1% of the entire drive, so space is not an issue.
each os is segregated via the partitioned virtual drives created when mint is installed. each non working os apps and files are accessible from each working os, so you can pull over gregarious files such as text, images, video ... and in the case of mint, openoffice will read and write all ms office files. so it's all pretty 'cross platform'.
hope some of this helps.