it sounds like (by what you've written) that you
wish to actually "triple-boot" where you have
Windows, Linux, and Linux as the OSes.

I've done this, using a 160 Gigabyte hard drive,
a separate boot partition and separate home partition to use with
the following operating systems:
1) Windows XP Media Center
2) Mint 32 bit
3) Mint 64 bit
I know this is an Ubuntu group, but in this case I ran Mint.

Anyway, I started out with only windows on the drive,
then I resized the windows partition, and divided up the rest
of the disk to use Linux partitions.

What wound up happening, is that once I installed Linux #1, it
was fine, it discovered Win Xp and did not do any damage to the MBR,
and I was able to boot into either XP or Linux at boot (using Grub).

Then, (this is where it got dicey) once I installed Linux #2 to the drive,
all broke loose in that only itself and Win XP were detected, ---Linux #1 was
not recognised, and this caused a new creation/installation of Grub so that
Linux #2 or Win XP could boot. It completely ignored Linux #1.

As a temporary workaround, I merely edited the GRUB file of Linux #2 to
make it aware of Linux #1.

Summary. This was a pain in the butt. Any kernel update to Linux #1 would
have to be accounted for manually in Grub of Linux 2.
And, from time to time (Not every boot mind you) the system would
experience Ubuntu Busybox errors where it could not find the particular
boot partition that it would need to find to boot the desired Linux OS.
Windows stayed out of this mess and was generally always able to boot up.

So, my advice is to be really careful how you set up the partitions.
Ideally, you'd want a GRUB file that was aware of both of your Linux OSes,
but a Single Grub file, not 2 of them, on the system. I am not sure of how this
can be done.