It's probably
replacing your grub.conf instead of
adding to it, so it forgets how to boot the previous OS.
You'll need to keep a copy of your grub.conf before installation, and re-add the relevant settings when something inevitably overwrites it, or worse, changes what partition grub looks for grub.conf in! That's why it's best to have a seperate /boot/, easier to keep track of what's booting from what, where.
Also, I don't understand your statement:
Quote:
1st installation of RedHat is succesful, but when it comes to 2nd installation of Fedora, it fails. It seems that they doesnt support the /, /usr/, /var etc to be cross platform.
Are you trying to make them all use the same /usr/, /var/, and so forth? Nono, give them their own!