I don't think you will be able to use JDB file everytime to update just AV definitions for the 20 endpoints.
Best option would be to make the 20 endpoints Managed clients and allowing them to take definition from Symantec Cloud (Internet).
Else you can have them as managed and promote any of the endpoint as GUP for Network B. Now GUP alone will take definitions from the SEPM in Network A. Rest of the endpoints in Network B will take it from the GUP in their same vicinity. There will be less administrative intervention needed in this setup.
As there are only 20 endpoints, any descent desktop should be able to perform as GUP. It should be connected to LAN and should be made to run 24X7 to ensure effective content distribution. Else you can resort to having a dedicated server with minimal configuration for GUP.
Hope this helps!