You can't have multiple devices on one mailbox. The reason being is that when you add a device to a BES and activate it creates a folder in the mialbox to contain BB info, like PIN/SRP/BES Name/etc. Each time you activate a new device it overwrites the current info in the folder.
The fiolder is a hidden folder and you cannot make any changes to it to use more than one device.
The ringer options are backed up to the BES and are part of the amilbox seeing that you are not removing the old device the folder just gets the updated PIN and BES info. When you remove a user from the BES the folder gets deleted. That folder provides core funtionality and should not be tampered with.
Programs like Outlook Spy or MFCMAPI can be used to view the folder but it's not easily readable. RIM posted an article on using MFCMAPI to pinpoint an activation issue with a users mailbox, if you want to see the folder, but don't go changing things. You might inadvertaintly ruin the mialbox.
This is how it works for Exchange, but all mail platforms can only have 1 device per mailbox. That's how it's designed.
The only thing you can do is have 1 device on BES and the rest on BIS, but I'm not sure how well a BIS account works with multiple hh's accessing it. It might not work that way either.
Hope that helps to clarify why it doesn't work.