you got three options then really. assuming that both boxes are being used on the same t,v. if seperate t.v's then its option 3
1. is to use yor existing feed for both the sky box and the freesat box, but means you'll only be able to use one box at a time and not together and will have to physically switch the cable from one box to the other.. not ideal
2. would be to get a satellite a/b switch
like this, but as above you'll only be able to use one box at a time, you wont have to physically swap cables but you will have to switch between boxes.
3. replace the single lnb (part on the end of the arm on the dish) for one with multiple outputs, either dual or quad usually, then connect your existing feed for sky to one of the lnb outputs, then run a new feed/cable from one of the other lnb outputs to your freesat box