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Unfortunately, you will not be able to do this, as this is a residential system, it was designed to be hassle-free for home users. Should you require some more conventional PBX functionality, you could use Euphoria Telecom.
Technically you could register the account on two devices at the same time however they will not both ring with inbound calls, they will re-register every hour depending on your phones setting. This means one might ring and the next time the other. You can, however, make outbound calls from both. It's not an ideal solution. You could look at getting a cordless DECT phone which will allow you to assign multiple handsets to a single line, that may be a solution for you.