Its easier to dual boot between win 7 & 10 if you use 2 hdd, having them both on same drive can be tricky.
I think if you set the boot method as legacy only, you can trick 10 into installing on a drive formatted to use win 7.
You need to install win 7 first, and then install win 10 afterwards into another partition, this should set up boot loader so it sees both OS
it might be easier again between 8 & 10 as you don't need to set them to legacy bios.