It is easy enough. But if you install it second, it will create the legacy boot manager, and not the new Windows 8 manager. What I do, when needed, is make an image of Windows 8. fwiw, I use Acronis. I then install Windows 7 on a second partition, and reinstate my Windows 8 image - best of both worlds.
But, maybe digressing, if you use the legacy Desktop in Windows 8, it has all the functions of Windows 7 and, with the addition of a third party Start menu, is little different in operation or appearance, from Windows 7. You then have the best of both worlds, without the need to reboot.