What you need is a router that has two FastEthernet interfaces that can be assigned IP addresses. The Cisco 2811 has two FE interfaces but may be overkill…I’m not familiar with any smaller models.
On interface FE1 assign it an IP address of 10.0.0.10 and connect it to LAN A modem. The login to LAN A modem and define a route 192.168.11.x to gateway 10.0.0.10.
On interface FE2 assign it an IP address of 192.168.11.10 and connect it to LAN B modem. Then login to LAN B modem and define a route 10.0.0.x to gateway 192.168.11.10.
This will enable both networks to “see” each other. As for sharing files and printers, you would probably need to setup a domain, like Microsoft Active Directory.