You can set up 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 FRITZ!Box you want to access.

Fig.: MyFRITZ! device overview on myfritz.net