i will try to give you some simple steps to get your box running with both distribuitions.
I know with Suse you can choose a directory where to install. So it might be best to first install redhat with an extra partition or big enough partition for suse. And after that start up the suse install routine and install in the choosen directory. Another nice thing of suse is that it makes space for itself by resizing existing partitions.
Another way would be to just make enough partition so you can install both distributions on diffrent partitions.
1.) Put in Redhat Cd.
2.) Boot Pc.
3.) Follow steps that are shown on screen.
4.) After Installation completion of Redhat, put in Suse CD.
5.) Reboot PC
6.) follow instruction.
7.) Install lilo or grub as the bootloader and it should automaticly show a menu at bootup from where you can choose which os to boot.
Hope it helps.