Each number has it's own log in and Backup Assistant account - so you'll need to register and create a log in for each number if it's not already done.
To edit the list for a particular phone, you'll need to log in with that phone number, edit the list, and then log out. Log in with the next number to edit THAT phone's contact list....etc. There is not a way, with one log in, to view all phones' contact lists. You can view usage, and features, and other things from the primary log in/account owner login. But Backup Assistant, Online Albums, Media purchases, etc are only viewable and manageable from the individual phone log in.
Hope that makes sense....