Use 2 of the 8 cables of an existing ethernet connection
If it is a CAT5 or CAT6 twisted pair cable, and it's used by 10baseT or 100baseT Ethernet (max 100 Mbit Ethernet), then the existing connection is only using pins 1,2,3 and 6 (usually the green and orange pairs) of the RJ45 plug. You can use the other 2 pairs (brown and blue, on the remaining 4 pins) as you like, even for a complete second Ethernet line, although they will interfere with each other (which, depending on the shielding, the speed and the length, may or may not make a difference.

There are probably better solutions for your application (like TCP/IP controlled relays), but this answers your question.