To Boot Windows 7 and Windows 8 from Two Hard drives or Two Partitions on a Single Drive.
Install Older OS first ie win7 before Win8..
Ensure in Power Saving options in Control Panel that Fast Booting Option is unchecked as this can cause selection problems.
Right click Computer and System Properties and Advanced System Settings
Startup & Recovery
Select OS Boot Options to set the default OS and the Timeout period default 30 seconds..
You may need a dedicated Mouse & Keyboard if using a KVM or KVMp setup as this will not be loaded at the selection stage.