First, do you have any experience in setting up any kind of Linux system?
Install the first operating system. As part of the process, set up the desired partitions** and leave free space for the other system.
Then install the second system on a new set of partitions** (swap can be shared). Most installers will automatically detect the first system and set up the boot manager. If not, you will need to do this manually.
For more detailed help, tell us the hardware specs and which versions of RedHat and Ubuntu you will be using.
Also tell us something about how the system will be used.
**There are many different ways to set up partitions. Do some Google searches if you don't already have a favorite.