I agree that no documentation seems to exist, at least online, that discusses the "legality" of doing what you're doing from a licensing standpoint. The unfortunate part about ArcGIS Server's licensing model is that there is no way to 'de-authorize' a server unless you uninstall the product and remove the key codes file. However, I would think that temporarily authorizing both the 10.0 and the 10.2.2 instances wouldn't be a big deal if (1) you're in the process of transitioning and using the 10.0 instance for reference while you configure the 10.2.2 instance as production, and (2) if users are only being directed to one of the instances at any given time. This is just my opinion, of course. Another concept I've heard about is the use of an EDN (Esri Developer Network) license. I'm not sure of the requirements for acquiring such a license, but you may be able to authorize your new 10.2.2 site with an EDN until it's truly ready for production; then authorize it with a production ECP number afterward followed by an uninstallation of ArcGIS Server from your old machine. You should probably check with Esri by asking Tech Support or your organization's Technical Advisor if one has been assigned to you.