In home chime mechanical or digital

In the event that someone’s house doesn’t have a front and back doorbell wired to a chime box ,…(though mine is) how can you link one doorbell to the doorbell that is wired to the chime box, to ring the chime when the doorbell that is not wired to the chime box is pressed,…Or if I wanted to put a doorbell on my gate,. And have it trigger the doorbell connected to the mechanical indoor chime. To ring the indoor chime.