Aleeve, sorry to say but this device will not split your internet. If you look at the enlarged image at Amazon, you can see a wiring diagram on top. It's not very clear, but you can see that the device uses all 8 pins of the RJ-45 connector to route the 4 input signals from the two 10/100-BaseT connectors. The pins of one connector are routed to the standard 10/100-BaseT pins and the pins from the other connector are routed to the 4 normally *unused* pins of the connector. Unless you have a device that recognizes this unusual wiring configuration, the second connector will just be ignored.
Note that the device is called a cable "economiser". One thing you can do with such a device is connect two different ethernet devices to a single cable that then makes a long run through a house or other building, then use another device to de-mux the two connections at the far end. Hence, you have used 1 cable for the long run instead of 2, ergo it is economised.
BTW, don't put such a device on a Gigabit ethernet (1000-BaseT) network -- those 8 pins all mean something unique on Gigabit ethernet.