To run two receivers off a single dish I think you would need a 'double' LNB - after that it's simply a case of running the new cable to your box.
Double LNBs seem to be easily obtainable - where I live you can pick them up in the equivalent of Comet although I must admit I've not seen them so commonly stocked in the UK.
One other thing to consider - and I don't know whether it is an issue - is the length of the cable run to your box.
Good luck.