Can I install two of these kits end-to-end using a single junction box?

If this is standard track channel, you can extend with connectors sold at Lowes. Usually you can get connectors to go straight, do a 90 and possibly a tee. These connectors snap into the track you have and are held in place with set screws. Be sure to shut down the circuit the light is on if already installed to avoid getting shocked or worse. They also make a cap that goes over the joint so it just looks like track.