The definition of a site to site VPN typically has instructions for a single subnet that can be found on the far side of that connection. If you have another subnet that you want reached on that far side, just make a route pointing to the ipsec map created by that VPN definition.. Below, I already created a site to site VPN with the name "site2" that already pointed to a network of 192.168.2.0/24. To reach another subnet on that side (192.168.3.0/24), I just add a route pointing to that ipsec map/name. I can do that for any number of subnets that I have on that side..
