how can do i install two linux os in one machine????
how do i partition the harddisk???

The way I would format the harddrive is this:

hda1-32M /boot
hda2-512M /swap
hda3- as needed
hda4- as needed

Install the os'es on 3 and 4, and put as much space on each as you wnat, the installer for each of the os'es should have a partioner.

Install the first distro on hda3 and let it install the bootloader on hda1. When you install the second one have it write a boot floppy.(Most distro's will overwrite grub or lilo on the boot drive if you just install SuSe and REdhat both did for me).

Boot to the distro which has it's bootloader on hda1 and go to the /grub folder and fild the file grub.conf and add an enter for the second distro so both boot off hda1. It should like like this:

title= (your distribution)
root (hd0,0)
kernel /kernel-(version) root=/dev/ram0 init=/linuxrc real_root=/dev/hda4
initrd /initrd(version)

the initrd and kernel version you will need to put in will be the same.

One more thing you will need to do is to copy the files from the boot floppy onto hda1.