Welcome to Flyertalk.
This is one of the most basic rules of FF programs and has been discussed 1000s of times almost in every airline forum in FlyerTalk.
Here are some examples:
http://www.flyertalk.com/forum/showt...mbine+accounts
http://www.flyertalk.com/forum/showt...mbine+accounts
http://www.flyertalk.com/forum/showt...mbine+accounts
And so on ...