RJ45 doubler that allows 2 computers on one plug

I have a 24 port old switch, but it uses 18 watts even when there is no traffic

Newer devices tend to consume less power than older ones.

I measured consumption of my D-Link 8-port Gigabit switch DGS-108. It consumes about 1W with a few devices connected.

With disconnected devices it consumes slightly less.

I also measured similar 5-port version DGS-105, and it consumed only slightly less than DGS-108.

One more thing to think about is opportunity cost (alternative cost). I like the idea to see our time as life currency. If you want you can spend hours choosing most efficient switch or one with least CO2 impact. Or you can spend hours planting trees. Or you can spends hours working more and then donating extra earned money to foundation which for example fights with climate change.

But you can't do all things, because a day has only 24h. Which means that by doing something you won't do something else in the same time.

I don't know which way is best, but I think after some experience you will notice that some ways to help planet may be more effective than others. And you may or may not come to conclusion, that it may be better to choose any switch now and spend this time doing something with more impact.

I have had an encounter with such a setup--although hardwired--some years ago. (Don't let electricians near a network wire!!) Despite low loads from the two computers involved it caused noticeable network lag.

I have my doubts that the RJ45 doubler you remember was only at one end. I don't see how it could do all these things with no power supply and nothing at the other end. I doubt, but I am not going to say it's impossible, I simply have never seen such a thing for Ethernet, and knowing what I know about Ethernet I can't imagine how it's possible. Anyways, you could go the hard-switch route. That makes it literal knob you must switch to share the one port. Good luck.

These can work by exploiting the fact that the cable has eight wires and only four are needed. So within the cable it splits onto the unused pairs. But they are double ended.

You say you are 100% sure that it was single ended. But in an answer you give your reason for being sure was that you replaced it with a normal ethernet cable. That is not proof it was single ended, because that would have used the default four wires, which are one of the two sets of four connected at the server end.

And of course that applies pre Cat 6 cable. Cat 6 uses all 8 wires anyway.