RHEL6 & UBUNTU installation?

1. Install RHEL6. When allocating disk space leave enough free space for the Ubuntu install.
2. Install Ubuntu.
3. When booting choose the appropriate option from the Grub boot menu.

You can save a little bit of space by using the same swap partition for each install. If you need several partitions for each install (perhaps you want separate mount points for /home, /boot, etc.) then create an Extended partition first covering the whole disk. Then you can create as many logical partitions as you require.