Yes, you can split an ethernet cable, as long as it's 10/100mb, but not gigabit, as that uses all 4 pairs.
10/100mb uses wire pairs 1/2 and 3/6. and most POE switches use the same wires for power (Mode A.)
Even if you are using the other pairs for power, it'll work as long as it's either both modeA or both ModeB.
(except that each cam gets the power from the other cam's feed.)
I have used a couple of splitters like these and they work fine for two cams in the same location, both POE:
http://www.ebay.com/itm/121878900894
I chose these because the seller clearly indicates the use case in a diagram:
be careful as some some splitter/combiners just connect all 8 wires together and are designed
to operate only one device at a time (not simultaneously.)
and of course, you'll need 4 more patch cables of the appropriate length to hook it all up.