You can either buy a
Quad LNB and run a second feed for your receiver, or you could follow this guide to switch the receiver into single tuner mode.
First make sure you are using single feed, check the back of your sky hd box and you will find that there will be only one cable connected to the dish input and that dish input 2 will be vacant
Press services on your remote control, "options" in the top will be highlighted orange
Press the right arrow once and highlight "Settings" in the top menu
Now press down once and go to the middle menu and highlight "picture"
Press "0,1, SELECT" on your remote quickly, This should highlight the setup box within the hidden menu
Now scroll down to the single feed mode
Press the right arrow once to switch single feed mode on
Now press the green button to save your settings
If you have done it properly you will get a message on screen saying "Your sky box is ready to be rebooted and selecting this option disables sky anytime"
Regards
Liam