Complex sentence: The two inboard shafts were driven by the high pressure ahead and astern turbines with the ahead turbines having an extra stage for cruising, this was separated from the main turbine by a bypass valve.
Split into simpler sentences: The two inboard shafts were driven by the high pressure ahead and astern turbines, with the ahead turbines having an extra stage for cruising. This was separated from the main turbine by a bypass valve.