How to connect the door bell
Installation of a doorbell in an apartment : How To Connect The Door Bell
Quote from Article entitled Installation of a doorbell in an apartment:
There is often a situation when you need to connect two buttons to one bell. This is necessary, for example, when there is a common tambour for several apartments. In this case, one is put in a tambour, and another one - in front of the door. The device connection circuit will then have two parallel buttons. Pressed any of them the circuit will close and the mechanism will operate.
The full version of the article can be found here.