Multiple RHEL installations

[Matthew Claridge]

This is going to sound a really stupid question, but how do I install 
more than one instance of RHEL on a single hard drive and then switch 
between them in Grub?

I've tried simply installing a second copy in some free space but it 
fails to see my first installation and if I add it to Grub manually it 
fails to boot from it.......This is somehting I haven't done for a long, 
long time and I'm confused.......

thanks in advance
Matt