You only need the one game. Install the one you have on your laptop, and enter your username and password as normal and thats it.
The only problem would be if someone broke into your home while you were on holiday and began playing WoW on your home PC at the same time you were playing it on your lappy lol. The same account cant be accessed (legally) more than once at the same time.
EDIT: Darn - beaten to the answer by milliseconds
