Separate HDDs for different OSs will work fine.
I previously (like 15 years ago) ran Ubuntu dual boot with a single HDD. In the end I found it a faff having to make a selection each time I powered on the PC - I disliked having to be involved in the boot procedure - and so from then til now I've run two computers, one running MSWindows, one running a Linux. It can be good to use both at once. I was doing that today. Windows for MS Word (which still works better than LibreOffice for docx files with tables) and Linux for quick web searches when I just need to check something. They're laptops, but the same can be done with desktop PCs if used with a bluetooth keyboard and mouse as long as each PC has a bluetooth receiver. That way you only need one keyboard and one mouse.
The key is not to store any of your files (i.e. your work) on either PC, but instead to use a USB drive. Just plug in the USB drive to whichever PC you're using for a given task. Fringe benefit of this is that you can easily take your files to any computer anywhere else (e.g. a PC in a workplace). I use a HDD in an external caddy but could use a USB thumb drive. Create backups regularly in case of USB drive failure (because we should be backing up our work files anyway). The PCs themselves can be used for creating and storing backups.
I realise this post is not an answer to the question you asked, but another way of achieving the same end. Forums, ay?
