i don't know if it can be used after or while both are in use, never tried that honestly
what i know is that i always have to make the phone call to activate the oem sticker license the first time if it can't be activated online, something that happened to me alot in 2009 and 2010
"Recently I rang Microsoft as I had a XP Home edition Disc, they kindly did a "One time only", generated 25 digit product code, for that CD only; as the original package had long ago gotten lost."
the one time only replaces the old key you had, that one if could be linked to the disc, will be invalidated of you try to activate it again, try to print that new one and keep using it on that machine, you got a free xp license
they are this gentle when it is a on the box license, if it is a oem, they won't help you much unless you insist alot, a friend had to do that and finally they gave him a new key but he said that he won't try that again, just make the owner buy another new