The main difference between 7 and 8 besides the RT/8 touchscreen gui and no longer seeing the Start button and menu is at the core level which includes some compatibility issues. For planning a dual boot if you go for the OEM type media release seeing 8 set up as a stand alone install on a totally separate drive and then later adding a boot entry into 7 is the best route. You can leave the 7 drive set as the default boot device.
The reason for that is running into a variety of problems that came up when booting into 7 from the 8 RP install. I can't say for sure that MS has fixed an apparent bug that may have lingered on into the retail release and why a stand alone is recommended.
For the second case I have here I ended up with a 7 Pro upgrade disk but not having any OS on a new build recently while still having older XP and Vista media on hand. The clean install was hesitant at first but was able to be activated there. With an 8 upgrade disk?
That's still up in the air at this time if the same would work since you now have to enter and watch as the product key is verified before the Windows installation will proceed. I haven't heard from anyone who has seen a clean from an 8 upgrade go and activate without the second install over the first required.