Good Afternoon,
Remember that is depending of the LAN on the other site, for example if a computer on your local LAN is 192.168.1.20 and this computer is trying to reach a computer with the IP address of 192.168.20.2 the router automatically is going to send the request to the correct tunnel, that is the reason why when you configure the site to site VPN you are request to enter the LAN of the remote site.
Regards.