

if it’s going to be on a separate SSD, it should be pretty straightforward: insert live USB for your secondary OS, go through its installer and be careful which SSD you select for it to install on (I recommend backing up all your important data before any procedure like this, just incase you make a mistake or something weird happens). and then you just need to make sure your bootloader knows about both OSes. the installer might automatically detect your existing OS and configure your bootloader to know about both. if not, you might have to update your grub config file after the installation is complete.

well, I just assumed Grub because in my experience, almost all Linux systems use Grub. I’ve never actually heard of Limine.
as for using different bootloaders for each operating system, that doesn’t really make sense to me. it’s technically possible to install different bootloaders on each SSD (and that might be what the installer does by default, if it doesn’t detect the presence of the existing operating system), but the point of a bootloader is to choose which operating system you want to boot, so if you set up a separate bootloader for each operating system, then you’d need a way to choose which bootloader you want to boot.
there actually is a way to do that, on most computers at least, you can use the BIOS/EFI boot menu to select which bootloader you want to boot. but I don’t think this approach really offers any advantages over having them share a bootloader.