I don't think you'll find a way to do this. There are three typical setups for registering with a sip server. You can either register the device itself to the server, which is the most common for typical home VoIP setups. The second most common option is to have every device register with a VoIP gateway that is on the premises (something like an Edgemarc router), which then registers all the devices on it at regular intervals, but all at the same time (rather than device by device whenever each device requires re-registration). The other types of setups are those that require a PBX, where the PBX manages registration by number, and not by device. The latter two are more common business setups.
What you want would have to be custom tailored and likely wouldn't be compatible with your SIP provider without their getting involved with part of the process. SIP is a session based protocol, so once a device authenticates and a session is created, anything else that uses those specific credentials is rendered useless as any other authentication attempt will fail while a valid one exists on the server. In some cases, depending on the application server, when another valid authentication attempt is made it is given precedence and the existing one is dropped. Even if you managed to proxy and route traffic correctly on your side, you'd still only be able to use one device at a time with only one set of credentials.