|Strictly speaking, it is not allowed under the licence. Here is an article that explains Windows 8 licensing, including that particular question - http://www.zdnet.com/how-the-new-wi...|
But that it is not in accord with the licensing conditions doesn't necessarily mean it is illegal, let alone criminal. Many licence terms are invalid in some countries. In particular many allow you to keep a copy of software as a backup, despite what licence terms may say.
In reality, you can do it and both OSs will remain activated (a different situation to trying to install on separate machines) which, I guess, means that Microsoft either can't tell or don't care. Morally - well, that's a moot point. Personally I would say that it's OK to keep the old copy long enough to ensure that you are happy with Windows 8. I suspect that in time you would find that you hardly ever use the old OS once you have everything running properly on 8 and would rather use the space than keep the old version.