You can configure your MyFRITZ! account in several FRITZ!Boxes. After registering an additional FRITZ!Box with MyFRITZ!, when you log into myfritz.net you will then see an overview of all the FRITZ!Boxes associated with your MyFRITZ! account. There you can select which of your FRITZ!Boxes you would like to access.

Fig.: MyFRITZ! device overview on myfritz.net