Since the satellite box has only one Scart it will have only one tuner. This is to be expected with the basic boxes procided under the digital switch-off scheme. To make use of another LNB you would need a second such box, available for some £70 or so. Most single boxes with twin tuners include hard drives (PVRs).
Although your Sony PVR and TV are described as digital, I assume that at the moment you are receiving analogue transmissions, otherwise you would not need to make any changes. If by digital you mean that they have Freeview tuners in-built, then presumably there is some reason why you are not currently using Freview or not intending to use when switch-over takes place.
It would be possible, for example, to use a Scart to composite video/S-Vhs converter to provide inputs to your recorder and TV from the one satellite box, provided you have the relevant input facilities. However, this would only facilitate recording the programme being watched.