The ICMP "Ping" command is for IP networks, not for ControlLogix backplanes. If the 1756-EN2T modules are physically or logically on different IP subnets, you will not be able to "PING" both of them.
However, RSLinx and similar software packages understand how to route through ControlLogix backplanes, so you can "drill down" using RSLinx to communicate with devices on the I/O network even with a computer that is on the enterprise network.
Browsing the I/O network requires that you know the IP addresses of those devices. In the RSWho browsing window, browse as far as the "Ethernet Network" underneath the I/O network's 1756-EN2T module.
Right-click and select Properties, and the "Browse Addresses" window will appear. Enter the IP addresses of the I/O network devices you want to communicate with.