How do you get alternate caller id for certain extentions

We have 2 businesses in one system and we want certain extensions to have 1 caller ID and another extension have another caller ID. There is an alternate caller ID tab but you can only chose between the main number and the extension phone numbers. We have another incoming line that is routed to certain extensions and set up as another site, but does not populate in the alternate Caller ID Options. How do I get these extensions to have an alternate Caller ID ?