I am in embedded development and so the question isn't regarding connectors, as we do not use RJ45s, but rather we use custom connectors of aerospace-grade.
Is it possible, and is it typically supported, to take a 1000BASE-T output (non-host interface) and split this to two different 100BASE-T outputs to two different network devices (e.g., so one ethernet interface becomes two from the kernel perspective in Linux).
Is this something conventionally supported by the 802.3 standard? Note these would be two entirely different network devices it would be connected to.
Then, in some cases based on the wiring configuration instead of two different 100BASE-T, it might just be a single 1000BASE-T to a single network device where high-speed is desired at the loss of an Ethernet interface.