It sounds like you probably used the wrong dualboot method when you
installed your OS's. Kent's idea is very good, but it won't work if you
installed your dualboot using the Microsoft method, which it sounds like
you must have done.
Multiboot methods fall into two general categories: the Microsoft way
and everyone else's way. The two methods use incompatible concepts and
cannot simply be substituted for one another. In a nutshell, the
Microsoft way intertwines the OS's by always booting through the same
partition and then forking to one or another operating system on
different drive letters, while the third-party boot managers keep OS's
totally independent and truly boot separate partitions as alternate "C:"
partitions.
The MS way intertwines the OS's and uses boot.ini to define the
available OS's in the boot menu. The third-party way does not
intertwine OS's and uses a separate boot manager, not the boot.ini file.
Since the third-party way doesn't intertwine OS's, it's easy to
completely hide or remove one OS while the other is booted, as Kent's
plan does. If you're using the MS boot loader (ntldr and boot.ini), you
can't do that.
You can't mix the methods by installing your OS's with MS's intertwining
(as you've done) and then try to use a third-party boot manager like
BootIt NG. So if you want to use Kent's technique you're going to have
to remove the second OS, restore/repair the first OS to single-boot
status, then reinstall the second OS using BootIt NG instead.