You need a gateway between the two networks. This is a basic feature of all network routers.
In the ControlLogix, you must make sure that both of the controllers have a Default Gateway configuration that points to the IP address of the gateway for their network.
When you configure the MSG instruction, you simply put in the IP address of the target controller. The router will take care of getting the message to the remote network and to the target device.
Are these two networks *physically* connected together in any way ? Are they VLANs, or just two logical networks that co-exist ?