So the setup I have is 1 harddrive with Windows 10 and 1 harddrive with Mint and batocera. If you already have Windows installed remember to check if it is GPT or msdos(MBR), otherwise the only way to boot from one to the other is from the bios boot menu. The way to check it is with the GParted tool from a linux USB.
I would start by having both harddrives connected to the computer, boot linux mint from a usb and start GParted. Check the Windows one for the partition table type and make sure the other one has the same. I would next make 1 partition of EXT4 for the Mint installation, make it the size you feel is necessary. Next I would make a 4gb Fat32 partition and label it BATOCERA. Form the size remaining I would make the SHARE partition, EXT4 format.
It is really important that BATOCERA and SHARE are in this order. It does not matter if they are in the beginning or end of the partition table.
Next close GParted and start the installer for the Mint system. Go through the menu and choose advance partition management (or whatever it is called) and put the / system on the first partition you made. It might complain about swap or /boot or something, do what it recommends. Finish the Mint installation, remove the USB and reboot. If all worked well you should have a grub screen and 10secs to choose between Mint and Windows.
Next, boot into Mint and mount the BATOCERA partition you made. If you can’t directly find it in the file manager, type disks in the start menu and start the Disks program. Choose the correct harddrive and partiton and click the “play” button to mount it. Next extract the contents of boot.tar.xz x86_64 into the boot partition. The partition should contain the folders boot, EFI, tools and batocera-boot.conf. Next run the commands from “Configure the grub boot loader” from the wiki. If all goes well you should now have Windows, batocera and Mint in the grub menu. Use the grub customizer to remove extra entries, change the time to choose/remove it and the order the systems are displayed.
I can’t really remember which order I did this but this is how I would do it now, should I have to do it again.