If you're installing a new disk that has enough space, you can simply make two (or three or four) partitions on that one hard drive. I once had one large hard drive with four OS's on it... long time ago, but I think I had Win98se, Win2k Pro, Win2k Server and some version of Linux all on one drive, but with four separate partitions, formatted with Fat32, NTFS and ext's for Linux.
The Linux OS was last to be installed, since I used Grub
to boot into the various operating systems. There was an OS on the disk previously and I think I used fdisk to remove it, and also made sure to "fdisk /mbr" to wipe the Master Boot Record. If you do it this way, you can set up Grub to ask on startup which OS to boot into, and set it up so that after a few seconds of inaction, it will automatically boot into any OS you've set up as the default OS.
It's a little more obtuse as far as initial setup, but once set up properly, this method has the advantage of not having to always bring up the BIOS setup to change default HDD for booting up, plus it's generally cheaper since you can use one hard drive for multiple operating systems.