I've never done this but it sounds intriguing. Are both images stored on another hard drive? If it goes belly up you could use your software to install the image again on the original drive. If the images are on the same hard drive you wish to try and dual boot, from the images, I wouldn't try it. Otherwise create a big partition on the drive now running win 7.
Boot from your software's rescue disk and install the image of Win 8 (from another drive or external drive) onto the new partition. Rebooting should give the option which OS to boot from, hopefully, otherwise I think 8 will take preference.
I'd have a go but I don't have much to lose as I've backups and images galore :) Thanks to the forumites here who remind from time to time the importance of backing up.