As said before, that works because both Sky and Freesat are transmitted from the same satellite.
TFC is transmitted from a completely different sateliite at a completley different location.
Andy, my simple testing this morning switching between two dishes and two boxes did not work out in my area. I did not make any adjustments to the set up of either dish.
I think the suggestion made by Imagine may offer a good solution.
It's no too expensive to add an additional independently adjustable LNB and have a specialist to set them up for you. Looking at my dishes and just "by eye" the actual spread between the two satellites looks like it's quite large. In which case the size of the dish will play a key part. You may need a much bigger dish.
Bottom line I think is to ask an installer to see what can be done with your existing arrangement (if anything) or just go for a new dish installation for Freesat.