Hello toppergraph, welcome to Seven Forums!
Have a look at the information below for some ideas and be sure to post back with any further questions you may have and to keep us informed.
The easiest way to do away with boot issues between separate Operating Systems (OS) is to use the BIOS one time boot menu to select which OS to boot at system startup, each motherboard has an individual hot-key to tap during system start-up to access this menu.
If you have 2 separate Hard Disk Drives (HDD) and have one OS installed to one HDD and you want to install another OS to the second HDD, disconnect the HDD with the first OS installed on it and leave only the HDD you want to install the second OS to connected.
Just be sure not to change where the original HDD SATA cable was connected, it has to be re-connected to the exact same port to avoid boot issues.
Install the second OS to the connected HDD and when complete and the system is booting good, power down and reconnect the first HDD with the first OS on it.
This way the OSs will boot independently of each other and there will be no boot conflicts between the 2 separate OSs to have to sort later.
Then set the BIOS to boot the HDD / OS you want as default and if you want to start the other (new) OS you use the BIOS one-time boot menu to select that HDD / OS to start when the PC is started.