As was mentioned, it us normally better to install the newer OS after the older OS.
But with windows 8/8.1 there was another configuration called UEFI. If you don't know what that is, then you need to do some research.
Then you need to decide if you want a Boot Menu, or pick a particular install to boot during the boot process.
I currently have Independent Windows 7 UEFI and Windows 8.1 UEFI install on different drives and it seems to work fine.
I don't believe you can use a Boot Menu with one UEFI and one Legacy install.