Dialplan Dual IAX Trunk (How do I Map calls between?)

I am adding an IAX trunk from a second trunk carrier. I am trying to figure out a good way to map phone numbers from one trunk to the other so that if a user with a phone number on IAX-A wants to call a number on IAX-B that the call will stall local and not travel down one trunk and back in the other. Does anyone have any examples of this or any idea on the best way to resolve this??

If you place a call from one IAX trunk to another on the same machine will it keep the call on the same machine or will it send it out?