Shared Line Appearance on Cisco 7941g IP Phone

We have two leads in our call center that management wants to be able to receive a hotline DN. They have a primary and secondary extension and this is a two line phone. When I add a third DN it won’t associate to the device unless I unassociate on of the existing dn’s. Supposedly this shared appearance rang on their phones before and would come in on one of their existing dn’s. I am having no luck. Any help is greatly appreciated.