Making an existing RHEL 8.0 system dual bootable with Debian 9.9

First, thank you to Christian and many others who helped me successfully perform a clean installation of RHEL 8.0.

I've just been told that I need to make this RHEL 8.0 system dual-bootable with Debian 9.9. Suggestions for the best way to do this? (I used the graphical interface for installing RHEL 8.0 and it worked smoothly; don't know if that info helps, but there it is.)

Again, new Linux user, so patience much appreciated.

Nina