I might have a couple of ideas/pointers for you, but first can I ask a question to clarify?
The impression I get is that you want to physically (ie lan-to-lan) join the eth0 network of each box (so . . the “far” ends of the connection), but the two networks are on different subnets.
Do you have a routing device in the picture somewhere?
I’ve implemented several instances of client-to-lan and also lan-to-lan, so I hope to be of help to you. Once you get a couple of concepts understood (well), zeroshell is so easy to work with . . almost a work of art!
Don’t give up on zs just yet.