In order to share files between two networks on the smae LAN you have to do routing table by loging into your router web base interface. You will need the subnet mask of each subnet/network. How many hosts does each subnet supports?
I'm in the exact same situation. I'm not quite sure what you mean by "do routing table", i know how to view my routing table from the command line. Can you give further info?
Each network has no more than 8 devices on it. so the subnets don't have to be too large.