Возможно ли запустить два подключения Ethernet по одному кабелю?

Is it possible to have one Cat5 (4-twisted-pairs) cable and split it into two Ethernet ports? I know Ethernet only needs 4 wires, so that would not be a problem, but would the connections interfere with each other or work without issues?

Yes, it will work, though it will be limited to 100 Base-T speeds. However, for the price of two splitters and the extra cables you would then need at each end, you could probably get a cheap Ethernet switch and keep your 1000 Base-T speeds.

There is a reasonable chance that it will drop down to 10 Base-T speeds. It is totally worth getting the Ethernet switch in my opinion.

You could use VLANs and give each “cable” a VLAN number so that computers set to a particular VLAN would only see packets tagged with that VLAN’s number along with using a standard (unmanaged) switch to split the cable into multiple ports.

You could also use a managed (business grade) switch that could handle the VLANs by itself and assign each VLAN to a particular port. That way, the computers do not need any configuration and it is a bit more secure since the computers would not receive any packets belonging to the adjacent VLAN. With the first situation, the packets still reach both computers, so if any of them are “evil”, they could still listen in on the adjacent VLAN’s traffic.