What do you mean by hidden, as in, after you boot your computer one operating system starts up while the second can only be accessed by a specific sequence?
I don't know if this is what you mean but:
I run windows vista and fedora core 7 on an hp laptop, i don't know the specifics of my boot configuration, but, when linux is beginning it's boot process, I can hit say, "Enter" or "Delete" and it brings me to an OS selector screen. From there I can chose from two versions of fedora and or/ windows(marked as 'other') like I said, I haven't exactly gotten the full hang of using fedora core, but it's still pretty the dominant system. So yes, you can boot two operating systems at the same time, in windows when the bios boot finishes it will give you the option to boot similar:
Windows XP
Windows Vista
Windows NT
etc.