With the secondary distributions installed, the steps to use them are:
It is important to use a 2.4-series or newer Linux kernel in the "primary" Linux distribution in order to take advantage of the multiple mount points feature that the 2.4-series permits. For many tasks, the /proc and /tmp filesystems will have to be remounted in the secondary distribution using:
mount --bind /proc /opt/distros/redhat_6.2/proc mount --bind /tmp /opt/distros/redhat_6.2/tmp
Also, it can be helpful to remount (rather than copy) the source tree from the primary to the secondary distro:
mount --bind /home/USER/src/PROJECT /opt/distros/redhat_6.2/USER/src/PROJECT
Become root and use:
xhost +localhost chroot /opt/distros/redhat_6.2 /bin/bash
Finally, create a user (if necessary) within the chroot-ed shell and develop, build, and/or test your application within this "separate" Linux distribution! Not that you may also have to specify your DISPLAY environment variable if you'd like to use X applications.
Done! You now have a shell that is, for all practical purposes, running within the secondary Linux distribution of your choice.