The devices that Mike posted are a 2-port ethernet switch powered by POE. Simple to use, and the one I have works well. Another method is to use the 4 otherwise unused wires in the
cat5/6 cable for a second link, using splitter/combiner devices at each end. Example here:
RJ45 Ethernet Cable Combiner/Splitter 1 Pair, 2-in-1 POE Cat6 Cat5e Data Adapter . Either approach will work in many situations, while sometimes one is a better solution. The 2-port switch is easier to hook up, but if the power required by the 2 cameras and the switch exceeds what the POE port can deliver, it won't work. Also, for a very long cable run, the splitter/combiner approach carries the power over twice as many wires, which could be an advantage.