Cable tester signals only 1, 3 and 6 for RJ45

For some reason our prebuild house has 2 RJ45 wall mounts in the same place (opposite sides of the wall, each has TV on it) but only single wire between them and router. So I was planning on splitting that single 8 pin wire into two 4 pin wires and have stable connection on both TV’s (we get only 100Mbps/s internet in our house so not a big deal). From outside cables looked like 8 pin ones, but when I tested it, for some reason, it showed signals only on 1, 3 and 6. Why that can be? I would expect 1, 2, 3 and 6 if it was already 100Mbps/s cable, but now im not quite sure about it.