Maybe is more clear , now…as sure you know , when a host has to send a packet to a network different that its own , it will send the packet to its default-gateway, so eg. when a host on the 192.168.1.0/24 network needs to send a packet to another host on 192.168.2.0/24 network , this packet will be sent to 192.168.1.1 , but if the default-gateway ,in its routing table has not a route to reach that destination , it will send this packet on its default-route ( ppp , dsl….).
I think that if you can manage your 192.168.1.1 and 192.168.2.1 routers , by adding a static route , on 192.168.1.1 something like:
ip route 192.168.2.0 255.255.255.0 via 192.168.1.(host address of local ZS)
and on 192.168.2.1 something like:
ip route 192.168.1.0 255.255.255.0 via 192.168.2.(host address of local ZS)
you could solve your issues.
As second solution , add on every host that needs to reach the remote private network a static route as above….