With the 7025, I have had one channel on the TV and another being viewed on the PC via VLC. I don't know about using the RF output as I've never used it.
This is really the wrong place for
Sky Plus, but to keep it brief it has two tuners but only one decoder. It records raw data and decodes at viewing time. This is completely different to PVRs such as the Dreambox which record what would be sent to the TV if you were viewing live. This a
Sky Plus can only ever output one thing at a time.