There is no way you could get a signal to the set upstairs as the second splitter would block the signal with the present setup.
You would need an RF modulator connected to the Scart on the TiVO, as this also feeds the downstairs TV it would have to be a loopthrough device like
THIS one. The feed going upstairs would have to be disconnected from the splitter and a female coax plug fitted, and a male plug fitted on the upstairs end going into the TV.