If I understood your question correctly:
Windows 8 automatically takes boot priority and Windows 7 will automatically become a optional boot choice in the Windows 8 boot os manager.
If you're talking about installation, you can split your main harddrive into another partition to install win 8 using the Windows 7 partition tool or a 3rd party program(I'd recommend MiniTool Parition Wizard;its free). But you'd need at least 21GB free for optimal usage.