You can't just "split it". You could connect an "ethernet switch" to the router though, & connect both the Sky box & Xbox to that.
Can you not just run another cable from the router to the Xbox?
Failing that, you could connect the Xbox wirelessly. The new "Slim" models (4GB, 250GB) have wireless built-in, the older models do not, however, so need an adaptor (additional cost).
Another option would be powerline adaptors. One would connect to the router, one to the Xbox, & the connection between the two would be via the mains.
Where are your Sky box, Xbox, & router located? Are they near each other or in different rooms?