Network Duplex – Full Duplex and Half Duplex Channel Capacity

Your thought process is based on faulty assumptions. By splicing 1 to 3 and 2 to 6 (which is what you have done), this does not put them in the same collision domain. Nor does this cause the devices to autonegotiate into half-duplex mode. Connect only one side of your main cable to one device and what you have now created is basically a "loopback."

Look at a hubbed 10BaseT or 100BaseTX environment. While they are in the same collision domain, they still have separate TX and RX pairs, they don't transmit in both directions on the same pair.

The reason for the collision domain is that a hub is ultimately a repeating device, which takes a signal in one port and repeats it out all other ports. If it receives traffic on two ports and repeats it out the other ports, the signal that is repeated will be a "collision" of the two signals creating an unusable signal. Thus the need for CSMA/CD and half-duplex operation.

By having three devices all spliced together in this fashion, you have merely created an environment that won't work. If you want to do this, simply put a hub between your two devices and connect a "listening" device to it. No need for a tap.

Shared physical media: only one device on the shared media can access it at any given time, the rest has to wait.

Halfduplex point to point links, uses the same physical media in both directions. It is the same frequency and point to point electromagnetic wave/channel. It is halfduplex as a radio can only transmit or receive at a given point in time. The two sides of the point to point link can not transmit at the same time they share the physical media

Multiplexing: is the interleaving of multiple data streams, the resulting multiplexed stream can then be sent over a full or half duplex link.