AltEPG

If I connect one box to the scart and one to the RF socket and set up Tivo for two providers, can I then record two channels at the same time or just record one channel and watch another?

The tivo can only record one thing at a time. You can watch what you are recording live, or any existing recordings already on the tivo.

To watch a second channel live you would need to bypass the tivo. e.g. You could let the tivo record from a sky box using the scart and use the RF loopthough to watch a freeview stb using RF as that won't involve the tivo. It won't be seamless and probably isn't what you have in mind.