Two receiver feed from one satellite cable

I have got sky hd box which is subscribed and watching hd channels ,I also got one sky free open box which plug in time by time on same frequency 28.8E, question is how can I feed both these receiver together as I have only one cable coming from satellite dish. Actually it's a dual while cable installed by sky.

I'm unable to install new cable to fit my purpose and need to know if I can install and plug/splitter/duplex in for both receiver. I just want to see one receiver at one time.

Pls comment.

Thanks