You cannot split a satellite feed due to the reasons noted by Allthegear, however, you can run both a Sky and a FreeSat decoders off the same dish if you purchase some additional equipment, namely a quad LNB and a MultiSwitch device.
Both are reasonably easy and cheap to install but please note that you will need to run 4 cables from the satellite dish instead of the existing one.
You replace the existing LNB installed on the dish with the Quad LNB, you then run 4 x Coaxial cables from this to the Multiswitch device. This is effectively a splitter which then allows you to connect as many decoders (Sky or FreeSat) to it as supported by the Multiswitch device you have bought (they come in many flavors) using a single coaxial cable for each decoder.
Please note that this will only work when looking at Astra satellite feeds used by FreeSat and Sky, for more exotic satellite feeds you will need additional dishes.
If you want I can provide the details of a couple of good Sat engineers based in the UK who I have worked with in the past who should be able to assist, just message me via my profile.