Technically, it would work, but since you are running 100BaseT on both sets, the amount of crosstalk is increased (also partially why the 2 pairs used for ethernet aren’t right next to each other in the connector). Interestingly, GigE uses all 4 pairs, but uses signalling that is less likely to cause crosstalk.

The fun part is that in theory, you could run 2 sets of ethernet at 10BaseT perfectly fine. At those speeds, the crosstalk should be minimal and the tolerances are much larger.

Another fun fact: There are many Cat levels for cable; another common one is Cat 3 which is only qualified for phone cabling, but happens to get used for ethernet frequently when the morons are being cheap.

And last but not least, if you want to drive an IT person nuts, sell them a Cisco 10/100 autonegotiating full-duplex switch and an Xserve G5. The NICs in Xserve G5s can’t negotiate correctly with the switch and will sporadically fail to get IP addresses or have other fits, especially during bootup. Netbooting is nearly impossible.

Each of the pairs has a different degree of twist to it, coinciding with their original purposes: one pair for network, one pair for phone, and the rest for serial I think. Yes, SERIAL. This standard comes from an age when people were as likely to have a dumb serial terminal on their desk as they were a phone and typewriter.

That was the origin of the standard ethernet jack. It sucked.