You need one tuner for each 'active' channel (i.e. one that you're either watching or recording).
Your recording device has two tuners for terrestrial digital TV (i.e. Freeview) built into it, so that, even if you'd not got Sky, you could use one tuner to watch one Freeview channel while using the other tuner to record a different one.
Adding your Sky box to your system adds a SINGLE tuner for satellite TV to what you've got. You can use that tuner to watch a Sky channel, or to record from it, but (because there is only one tuner) you can't watch satellite TV and record from it at the same time.
Chris