You could get a simple Y co-axial connector. Incomming cable into one end a the two outgoing into the other end, simple as that, the connector should only cost a few quid.

However if you splice the signal in two you also loose strenght in the signal, so if you were willing to spend a few quid more you can get a booster/splitter unit (you can buy them in argos not that expensive) this unit allows you to split and amplify the signal for up to 5 or 6 ways.
You need power for this unit so you'll need to have it close to a socket.

Hope this helps.

If I get a chance I'll draw a diagram and PM it to you .