You simply need a push route statement in each opposing server to route to the other subnet.
So, for the 192.168.89.x server, add:
push 'route 192.168.88.0 255.255.255.0'
And for the 192.168.88.x server, add:
push 'route 192.168.89.0 255.255.255.0'
Make sure you've got IP forwarding enabled on the server so that it will route between the interfaces.