You didn't fill out your specs, so I don't know what kind of computer that you have or how many hard drives are available, but the only problem that I ever had were due to the boot loader going bad, particularly when the OSs were in different partitions of the same hard drive. I now always install each OS on separate drives, while all other drives are disconnected, until the installation is complete, then install the other OS in like manner on another hard drive.
Doing this eliminates the boot loader, an insures that one OS is always available, unless there is a massive hardware failure. The only difference is that instead of using a boot loader, you have to use the F8 BBS Popup Menu, to select a hard drive to boot, instead of an OS.
It sounds as though you already have the OSs installed (probably with a boot loader, so I would not change that...until it becomes necessary to reinstall one of the OSs...if you have multiple hard drives.