Answering at the 30,000 foot level, there are two obvious options, with examples of both already posted:
1. A poe powered 2 (or more) port ethernet switch.
2. A splitter-combiner set that places a 2nd ethernet channel on the otherwise unused 4 wires of an existing ethernet link.
Option 1 is a bit simpler to hook up, saves a port on your POE switch, but requires that port to power 2 cameras and the poe powered switch. The single port must be capable of powering both cameras.
Option 2 works the same as if you has run 2 ethernet cables, with added connection complexity on both ends. In general, works only with POE switches, not injectors, because injectors tend to use all 4 pairs on the ethernet cable. Also not usable with POE++ which also uses all 4 pairs.
I've used both. They both work. Some people think that running 2 links on a single cable compromises data integrity. I've not seen any hints of that in my system.