One way you could do it is have both phones have their own extension and the one you want to share, you create a user for then use MADN appearances which work similar to Bridged. Any external number can have an ICR record set to the "Phantom" or "Shared" extension number, then both phones have appearances for the Shared extension, along with their own respective call appearance keys. I would use MADN Single and make one for each appearance (eg. a, b, c). MADN multiple could be a bit more confusing for the users.
MADN is a great feature of R11.
Thanks, Tim
Adelaide, Australia