Complex sentence: The boilers had a combined heating surface of and supplied steam at 425 lb f/in 2 to two turbo generators supplying two electric motors with a combined rating of 6,315 NHP or 28,000 shp.
Split into simpler sentences: The boilers had a combined heating surface of and supplied steam at 425 lb f/in 2 to two turbo generators. These supplied current to two electric motors with a combined rating of 6,315 NHP or 28,000 shp.