Problem connecting two databases from two different networks connected through VPN(only TCP 1521 ope

Folks,

I have two networks that work independently. Each network has a server hosting a DB (11g) and another hosting the connection manager that funnel all the connections to the network using port 1521. Clients access the db internally on port 1521. access through the internet is blocked by the firewall.

I need to connect the two databases together. The two networks are connected through a VPN connection between specific servers only and on port 1521 only.

As a recap

NETWORK 1 < ---- PORT 1521 --- > CMAN 1.1 (server 1)  < ---- ports handled by oracle --- > DB 1 (server 2) < ---- ports handled by oracle --- > CMAN 1.2 ( server 3)

NETWORK 2 < ---- PORT 1521 --- > CMAN 2.1 (server 4)  < ---- ports handled by oracle --- > DB 2 (server 5)