BZZ There are videos about it on youtube.
Basically you can boot a batocera image without making a new partition.
You install grub2win and you create a folder \batocera on the root of your drive, you put the batocera img file and their gz file in the same folder. They also have a pre-configured grub.cfg file you have to edit to point towards the path of your image file. When booting your computer you choose between win 10 or the batocera image and it will boot it.
I thought I had everything sorted out until I restored my roms and activated the Nvidia driver then it wouldn't boot the image anymore.
I'm getting a second hard drive so I will have win10 on one drive the batocera on the other. Maybe then I can finally dual boot.
Never thought it would take such an amount of effort to simply dual boot both systems. Ugh...