I would be very cautious about using power line adapters, they are known to generate interference at the frequencies used by the VDSL2 connection.
How about a much simpler solution provided that your existing Ethernet cable is the normal 4 pair type you can use this to provide 2 100Mbit connections as each one uses 2 pairs. So all you need is 4 RJ45 connectors and a crimp tool then check the RJ45 ethernet wiring diagram to use the appropriate pairs at each end of the cable.
You can get ready made adapters called cable economisers but these seem to have female RJ45 sockets so extra patch cables and couplers are needed to connect to the modem/router etc.
Edited by Realalemadrid (Wed 22-Mar-17 08:52:26)