Complex sentence: These assume slow net separation of continental crust from mantles that are still mostly unfractionated and are vigorously convecting in similar bottom-driven fashions; but also that this common process yields altogether different surface effects on each planet.
Split into simpler sentences: These assume slow net separation of continental crust from mantles that are still mostly unfractionated and are vigorously convecting in similar bottom-driven fashions. But they further assume that this common process yields altogether different surface effects on each planet.