I will confess. I started writing this post about some stupid systemd tricks. However, I wanted to explain a little about systemd first, and that wound up being longer than the tricks. So this Linux ...
I've got an existing Arch64 install, with win10. EFI, systemd-boot. From the looks of /boot, it seems that another linux install on the same ESP would try and overwrite the existing vmlinuz etc files ...