This is possible by creating a dummy extension then adding this dummy extension to the Paging group of the system sending the page. Then within mobile phone field of this dummy extension you would specify the prefix used to dial the other side of the bridge and the page group (virtual extension number) of the bridged system, then from the forwarding rules of the dummy extension, set 'Ring Mobile My Simultaneously'. Now when you dial the page group you will also page the page group of the bridged system, which you would create and add the extensions of the bridged system into.