If I've understood your query correctly (if not, apologies):
For 10/100Mbps Ethernet transmission just 2 of the 4 pairs are used - but over the single wire coax the signals are modulated on to multiple carriers.
Think of a TV signal over coax - how many channels can it transmit, when each is modulated over a carrier at a slightly different frequency than the others, so you can tune each in selectively.
Ethernet signals over UTP Unshielded Twisted Pair cable are not modulated on to separate carriers.