Cat5 and under only use 4 wires (2 pairs) and while technically Cat5e and Cat6 use all 8 wires (4 pairs), in the case of security cameras it's not necessary (considering some of the most demanding cameras barely push 20MB/s).
I'm assuming you're trying to be thrifty and use 1 Ethernet cable for 2 cameras (4 wires/2 pairs each)?