I've done a similar thing, but split it into 2 ethernet. It's a bit tricky to sort out the wires you will be using and get it right on both ends, and hopefully no one else will ever need to know what you did because it is definitely not considered spec to do it this way. I've never had any issues as far as functionality though, once I got it working properly. It will only link at 100M/bit which I've always attributed to the fact it was rigged up this way, but doesn't really cause any issues.
10/100Mbit only use 2 pair of wires. The other two pairs are used to prevent crosstalk on the line. 1000Mbit requires all 4 pair, which is why you are only getting 100mbit speeds.Gigabit Ethernet uses all 4 pairs (8 conductors). The transmission scheme is radically different from 10 and 100 Mbit/s standards (PAM-5: a 5 level amplitude modulation scheme) and each conductor is used for send and receive.