Freeview boxes only contain a single Freeview decoder. If you want to watch one channels whilst recording another, you would be best advised to get a Personal Video Recorder or a PVR-DVD writer combo, as these provide simple access to recording.
Freeview boxes do not have an "RF output" so you cannot "tune" into their decoded signal.
Would a PVR or PVR-DVD replace my set-top box or be used with it?
A PVR or PVR-DVD would replace your set-top box - they have at least two Freeview tuners built-in. The 'old' set-top box could be used with another TV, however.