yes , if your main is already awakened and purchased the item after awakening ( stone dissapears when you are done with awakening quest even if you didn't use it) your subclass which you choose to do dual class quest with is not awakened yet
source - https://forum.4game.com/showthread.php?t=8606&page=3&p=74424&viewfull=1#post74424 therefore if your dual subclass is still 3rd class and has not gone through the awakening process then yes the stone can be used then. the item is not storable/tradable etc therefore you can not store it in your warehouse and it will disappear once you have awakened. regards. thread moved to archives.