The two networking can work at the same time, and you need to connect two adapters at both end. 

Cat6 RJ45 8P8C plug to dual RJ45 splitter, network Ethernet patch cord adapter with shield

This compact splitter is the perfect tool for expanding your home or office network, allowing more than one computer to be connected to the same ports

Easily convert your RJ45 outlet to two RJ45 sockets with this network splitter

This compact design allows you to comfortably increase the number of RJ45 network connections on an RJ45 outlet

Allow two computers to share high speed DSL, cable modem and Ethernet ports