Are they on the same drive?
If they on two drives, when you install win 8 it might write over win 10's boot sector, its generally a better idea to install the OS in the other order (8 then 10) as older versions don't know how the newer versions work or even exist and can over write essential files. There are ways to do it though, just need to rebuild win 10 MBR
Can you supply a screen shot (share to imgur and link here) of disk management and we can see how it will go.