Connect the two laptops together using the Ethernet cable. Connect the audio outputs of each interface to the channel inputs on your mixer. Try playing music from both machines at once, and panning between them using the cross-fader. Set the volume levels from each to be similar, and adjust the mixer output level to a comfortable volume.
Connect headphones to the socket on the audio interface, and try using the output select controls to listen the different sources. Depending on which DJ software you are using, this may be done by a switch, or a blend dial. You won't be able to switch between sound from both laptops yet, just between the two virtual turntables on each machine.
Access the "Preferences" menu on your "Channel 1" laptop and locate the "MIDI Options" section. Enable "MIDI Clock Send" and select your Ethernet cable as the route. This will cause your first laptop to send a signal of its internal clock pulses to the second laptop, allowing them to synchronize with each other.
Access the "Preferences" menu on your "Channel 2" laptop and enable "MIDI Clock Receive" --- it may be referred to as "Slave Mode" depending on which software package you are using.
Play a song on the first laptop, then attempt to use the a tempo-synced effect on the second laptop; they should both match up with the beat. If there is any delay with the timing, access the "preferences" again and adjust the "MIDI Clock Offset" control until the signals match up.